Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	$ 27,199	$ 27,427
Gross unrealized gains	2,133	1,747
Gross unrealized losses	(240)	(418)
Fair value	29,092	28,756
U.S. government and agencies
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	683	678
Gross unrealized gains	96	90
Gross unrealized losses		(2)
Fair value	779	766
Municipal
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	3,103	3,135
Gross unrealized gains	334	231
Gross unrealized losses	(36)	(62)
Fair value	3,401	3,304
Corporate
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	20,552	20,397
Gross unrealized gains	1,482	1,214
Gross unrealized losses	(161)	(295)
Fair value	21,873	21,316
Foreign government
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	703	715
Gross unrealized gains	84	83
Gross unrealized losses	(3)	(6)
Fair value	784	792
Asset-backed securities ("ABS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	771	1,011
Gross unrealized gains	28	30
Gross unrealized losses	(28)	(34)
Fair value	771	1,007
Residential mortgage-backed securities ("RMBS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	703	752
Gross unrealized gains	56	50
Gross unrealized losses	(8)	(12)
Fair value	751	790
Commercial mortgage-backed securities ("CMBS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	670	724
Gross unrealized gains	51	47
Gross unrealized losses	(4)	(7)
Fair value	717	764
Redeemable preferred stock
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	14	15
Gross unrealized gains	2	2
Fair value	$ 16	$ 17