Investments (Unrealized Losses and Fair Values) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	$ 5,928	$ 6,838
Less than 12 months, Unrealized losses	(66)	(226)
12 months or more, Fair values	4,621	4,360
12 months or more, Unrealized losses	(61)	(115)
U.S. Treasury notes [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	1,190	1,219
Less than 12 months, Unrealized losses	(11)	(21)
12 months or more, Fair values	1,452	1,444
12 months or more, Unrealized losses	(16)	(26)
Other government-related debt securities - U.S. [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	0	0
Less than 12 months, Unrealized losses	0	0
12 months or more, Fair values	111	110
12 months or more, Unrealized losses	(1)	(2)
Other government-related debt securities - Foreign and other [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	410	631
Less than 12 months, Unrealized losses	(6)	(31)
12 months or more, Fair values	307	240
12 months or more, Unrealized losses	(4)	(10)
Corporate debt securities - Financial [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	1,783	1,968
Less than 12 months, Unrealized losses	(14)	(59)
12 months or more, Fair values	822	718
12 months or more, Unrealized losses	(8)	(22)
Corporate debt securities - Industrial [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	1,516	1,898
Less than 12 months, Unrealized losses	(20)	(81)
12 months or more, Fair values	634	529
12 months or more, Unrealized losses	(7)	(18)
Corporate debt securities - Other [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	454	529
Less than 12 months, Unrealized losses	(6)	(20)
12 months or more, Fair values	36	28
12 months or more, Unrealized losses	(1)	(1)
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	558	576
Less than 12 months, Unrealized losses	(9)	(14)
12 months or more, Fair values	809	840
12 months or more, Unrealized losses	(13)	(22)
Other mortgage- and asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	17	17
Less than 12 months, Unrealized losses	0	0
12 months or more, Fair values	450	451
12 months or more, Unrealized losses	$ (11)	$ (14)