Slicers (applies to each fact value in each table cell)

Reporting Entity [Axis]

0000049071 (http://www.sec.gov/CIK)

Fair Value Disclosures [Abstract]	Period [Axis]
	2016-01-01 - 2016-12-31
Fair Value Disclosures [Abstract]
Financial Assets Measured at Fair Value on Recurring Basis	The following table summarizes our fair value measurements at December 31, 2016 and 2015, respectively, for financial assets measured at fair value on a recurring basis:       Fair Value Measurements Using   Fair Value   Quoted Prices in Active Markets (Level 1)   Other Observable Inputs (Level 2)   Unobservable Inputs (Level 3)   (in millions) December 31, 2016               Cash equivalents $ 3,654   $ 3,654   $ —   $ — Debt securities:               U.S. Treasury and other U.S. government corporations and agencies:               U.S. Treasury and agency obligations 786   —   786   — Mortgage-backed securities 1,637   —   1,637   — Tax-exempt municipal securities 3,305   —   3,302   3 Mortgage-backed securities:               Residential 9   —   9   — Commercial 304   —   304   — Asset-backed securities 160   —   160   — Corporate debt securities 3,597   —   3,593   4 Total debt securities 9,798   —   9,791   7 Total invested assets $ 13,452   $ 3,654   $ 9,791   $ 7 December 31, 2015               Cash equivalents $ 2,229   $ 2,229   $ —   $ — Debt securities:               U.S. Treasury and other U.S. government corporations and agencies:               U.S. Treasury and agency obligations 332   —   332   — Mortgage-backed securities 1,891   —   1,891   — Tax-exempt municipal securities 2,668   —   2,663   5 Mortgage-backed securities:               Residential 13   —   13   — Commercial 985   —   985   — Asset-backed securities 263   —   263   — Corporate debt securities 2,958   —   2,952   6 Total debt securities 9,110   —   9,099   11 Total invested assets $ 11,339   $ 2,229   $ 9,099   $ 11
Changes in Fair Value of Assets Measured Using Significant Unobservable Inputs	During the years ended December 31, 2016, 2015, and 2014, the changes in the fair value of the assets measured using significant unobservable inputs (Level 3) were comprised of the following:   For the years ended December 31,   2016   2015   2014   Private Placements   Auction Rate Securities   Total   Private Placements   Auction Rate Securities   Total   Private Placements   Auction Rate Securities   Total   (in millions) Beginning balance at January 1 $ 6   $ 5   $ 11   $ 24   $ 8   $ 32   $ 24   $ 13   $ 37 Total gains or losses:                                   Realized in earnings —   —   —   (1 )   —   (1 )   —   —   — Unrealized in other comprehensive income —   —   —   —   —   —   —   —   — Purchases —   —   —   —   —   —   —   —   — Sales —   —   —   (17 )   (3 )   (20 )   —   (5 )   (5 ) Settlements (2 )   (2 )   (4 )   —   —   —   —   —   — Balance at December 31 $ 4   $ 3   $ 7   $ 6   $ 5   $ 11   $ 24   $ 8   $ 32