Investments and Cash - Net Realized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Realized Investment Gains (Losses)
Gross realized gains on available-for-sale securities									$ 28	$ 44	$ 14
Gross realized losses on available-for-sale securities									(8)	(15)	(5)
Net realized gains (losses) on other invested assets									2	(8)	6
Other-than-temporary impairment									(51)	(47)	(75)
Net realized investment gains (losses)	$ (24)	$ (2)	$ 10	$ (13)	$ (6)	$ (27)	$ (9)	$ 16	$ (29)	$ (26)	$ (60)