Investments (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	$ 23,112	$ 23,770
Gross unrealized gain	1,310	1,321
Gross unrealized loss	(200)	(462)
Fair value	24,222	24,629
U.S. government and agencies
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	968	920
Gross unrealized gain	48	57
Gross unrealized loss	(2)	0
Fair value	1,014	977
Municipal
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	2,017	2,162
Gross unrealized gain	264	292
Gross unrealized loss	(7)	(12)
Fair value	2,274	2,442
Corporate
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	18,945	18,069
Gross unrealized gain	905	849
Gross unrealized loss	(169)	(414)
Fair value	19,681	18,504
Foreign government
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	304	348
Gross unrealized gain	28	36
Gross unrealized loss	0	0
Fair value	332	384
ABS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	337	1,443
Gross unrealized gain	4	5
Gross unrealized loss	(10)	(28)
Fair value	331	1,420
RMBS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	294	406
Gross unrealized gain	42	49
Gross unrealized loss	(3)	(4)
Fair value	333	451
CMBS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	233	409
Gross unrealized gain	17	31
Gross unrealized loss	(9)	(4)
Fair value	241	436
Redeemable preferred stock
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	14	13
Gross unrealized gain	2	2
Gross unrealized loss	0	0
Fair value	$ 16	$ 15