Investments (Unrealized Losses and Fair Values) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 20,139	$ 25,250
Less than 12 months, Unrealized losses	(468)	(191)
12 months or more, Fair value	1,370	2,061
12 months or more, Unrealized losses	(40)	(31)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	5,023	7,728
Less than 12 months, Unrealized losses	(103)	(70)
12 months or more, Fair value	53	195
12 months or more, Unrealized losses	(1)	(2)
Other government-related debt securities - U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	103	188
Less than 12 months, Unrealized losses	(2)	(1)
12 months or more, Fair value	26	34
12 months or more, Unrealized losses	(1)	(1)
Other government-related debt securities - Foreign and other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,367	1,163
Less than 12 months, Unrealized losses	(39)	(9)
12 months or more, Fair value	88	115
12 months or more, Unrealized losses	(4)	(2)
Corporate debt securities - Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	5,648	5,928
Less than 12 months, Unrealized losses	(128)	(28)
12 months or more, Fair value	325	462
12 months or more, Unrealized losses	(9)	(6)
Corporate debt securities - Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	4,702	5,760
Less than 12 months, Unrealized losses	(117)	(43)
12 months or more, Fair value	397	612
12 months or more, Unrealized losses	(11)	(9)
Corporate debt securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	727	868
Less than 12 months, Unrealized losses	(18)	(4)
12 months or more, Fair value	93	117
12 months or more, Unrealized losses	(3)	(2)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,464	1,838
Less than 12 months, Unrealized losses	(39)	(24)
12 months or more, Fair value	226	276
12 months or more, Unrealized losses	(7)	(6)
Other mortgage- and asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,105	1,777
Less than 12 months, Unrealized losses	(22)	(12)
12 months or more, Fair value	162	250
12 months or more, Unrealized losses	$ (4)	$ (3)