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2412404 - Disclosure - Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details)
(http://www.alliantenergy.com/role/BenefitPlansDefinedBenefitPensionAndOtherPostretirementBenefitsPlansDetails)
TableSchedule of Defined Benefit Plans Disclosures [Table]
Slicers (applies to each fact value in each table cell)
Reporting Entity [Axis]0000352541 (http://www.sec.gov/CIK)
Defined Benefit Plan Disclosure [Line Items]Period [Axis]
2016-01-01 - 2016-12-31
2015-01-01 - 2015-12-31
2014-01-01 - 2014-12-31
Legal Entity [Axis]Legal Entity [Axis]Legal Entity [Axis]
IPL [Member]WPL [Member]Entity [Domain]IPL [Member]WPL [Member]Entity [Domain]IPL [Member]WPL [Member]Entity [Domain]
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis]
Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]Defined Benefit Pension Plans [Member]Other Postretirement Benefits Plans [Member]
Service cost
7,500,000  
2,300,000  
4,900,000  
2,000,000  
12,600,000  
5,300,000  
8,800,000  
2,400,000  
5,800,000  
2,100,000  
15,900,000  
5,500,000  
7,200,000  
2,400,000  
4,900,000  
2,000,000  
13,100,000  
5,200,000  
Interest cost
24,500,000  
3,800,000  
22,300,000  
3,800,000  
53,000,000  
9,400,000  
25,000,000  
3,800,000  
22,600,000  
3,700,000  
53,600,000  
9,100,000  
25,100,000  
3,900,000  
22,600,000  
3,800,000  
54,100,000  
9,500,000  
Expected return on plan assets
(30,900,000)1
(4,300,000)1
(28,300,000)1
(800,000)1
(65,500,000)1
(6,100,000)1
(35,800,000)1
(5,700,000)1
(32,400,000)1
(1,500,000)1
(75,000,000)1
(8,400,000)1
(35,700,000)1
(5,800,000)1
(32,400,000)1
(1,300,000)1
(74,900,000)1
(8,300,000)1
Amortization of prior service cost (credit)
(200,000) 
(2,600,000)2
200,000  
(900,000)2
(300,000) 
(4,100,000)2
(100,000) 
(6,100,000)2
200,000  
(3,500,000)2
(200,000) 
(11,300,000)2
0  
(6,300,000)2
300,000  
(3,900,000)2
0  
(11,900,000)2
Amortization of actuarial loss
16,500,000 3
2,600,000 3
17,600,000 3
1,800,000 3
37,400,000 3
4,700,000 3
15,300,000 3
2,300,000 3
16,800,000 3
2,200,000 3
35,400,000 3
4,800,000 3
8,000,000 3
1,100,000 3
9,200,000 3
1,300,000 3
19,500,000 3
2,400,000 3
Additional benefit costs
  
0  
0  
0  
0  
  
500,000  
0  
500,000  
0  
  
0  
0  
0  
0  
Total
17,400,000  
 
1,800,000  
 
16,700,000  
 
5,900,000  
 
37,200,000  
 
9,200,000  
 
13,200,000  
 
(3,300,000) 
 
13,500,000  
 
3,000,000  
 
30,200,000  
 
(300,000) 
 
4,600,000  
 
(4,700,000) 
 
4,600,000  
 
1,900,000  
 
11,800,000  
 
(3,100,000) 
 
1: The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
2: Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan.
3: Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.