Investments - Fair Values (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	$ 21,057	$ 22,004
Gross unrealized gain	732	1,355
Gross unrealized loss	(389)	(98)
Fair value	21,400	23,261
U.S. government and agencies
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	740	768
Gross unrealized gain	33	38
Gross unrealized loss	0	(2)
Fair value	773	804
Municipal
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	1,997	2,001
Gross unrealized gain	202	275
Gross unrealized loss	(4)	(3)
Fair value	2,195	2,273
Corporate
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	17,521	18,262
Gross unrealized gain	433	960
Gross unrealized loss	(381)	(86)
Fair value	17,573	19,136
Foreign government
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	170	279
Gross unrealized gain	9	20
Gross unrealized loss	0	0
Fair value	179	299
ABS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	429	383
Gross unrealized gain	3	6
Gross unrealized loss	(3)	(4)
Fair value	429	385
RMBS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	154	205
Gross unrealized gain	44	49
Gross unrealized loss	(1)	(1)
Fair value	197	253
CMBS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	33	93
Gross unrealized gain	7	6
Gross unrealized loss	0	(2)
Fair value	40	97
Redeemable preferred stock
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	13	13
Gross unrealized gain	1	1
Gross unrealized loss	0	0
Fair value	$ 14	$ 14