Investments (Net change in unrealized gains (losses) on investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	$ 758	$ 224	$ (1,119)
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	728	225	(1,114)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	32	(2)	(6)
Other
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	$ (2)	$ 1	$ 1