Schedule of Debt | | | | | | | | | | | December 31, 2016 | | December 31, 2015 | | (Millions) | Amended and Restated Credit Agreement | | | | Revolving credit facility, weighted-average variable interest rate of 2.01% and 1.57%, as of December 31, 2016 and 2015, respectively, due May 1, 2019 | $ | 195 |
| | $ | 375 |
| Debt Securities | | | | Issued November 27, 2012, interest at 2.50% payable semi-annually, due December 1, 2017 | 500 |
| | 500 |
| Issued March 13, 2014, interest at 2.70% payable semi-annually, due April 1, 2019 | 325 |
| | 325 |
| Issued March 13, 2012, interest at 4.95% payable semi-annually, due April 1, 2022 | 350 |
| | 350 |
| Issued March 14, 2013, interest at 3.875% payable semi-annually, due March 15, 2023 | 500 |
| | 500 |
| Issued March 13, 2014, interest at 5.60% payable semi-annually, due April 1, 2044 | 400 |
| | 400 |
| Unamortized issuance costs | (11 | ) | | (14 | ) | Unamortized discount | (9 | ) | | (12 | ) | Total debt | 2,250 |
| | 2,424 |
| Current maturities of long-term debt | 500 |
| | — |
| Total long-term debt | $ | 1,750 |
| | $ | 2,424 |
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Future Maturities of Long-Term Debt | The future maturities of debt in the year indicated are as follows: | | | | | | Debt Maturities | | (Millions) | 2017 | $ | 500 |
| 2018 | — |
| 2019 | 520 |
| 2020 | — |
| 2021 | — |
| Thereafter | 1,250 |
| | 2,270 |
| Unamortized issuance costs | (11 | ) | Unamortized discount | (9 | ) | Total | $ | 2,250 |
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