Investments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	$ 27,796	$ 41,194
Gross unrealized gains	1,931	4,108
Gross unrealized losses	(392)	(426)
Fair value	29,335	44,876
U.S. government and agencies
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	705	2,137
Gross unrealized gains	110	242
Gross unrealized losses	(1)
Fair value	814	2,379
Municipal
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	3,184	4,153
Gross unrealized gains	273	612
Gross unrealized losses	(47)	(61)
Fair value	3,410	4,704
Corporate
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	20,545	28,748
Gross unrealized gains	1,320	2,896
Gross unrealized losses	(270)	(113)
Fair value	21,595	31,531
Foreign government
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	726	1,017
Gross unrealized gains	94	164
Gross unrealized losses	(4)	(1)
Fair value	816	1,180
Asset-backed securities ("ABS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	942	1,921
Gross unrealized gains	32	49
Gross unrealized losses	(36)	(105)
Fair value	938	1,865
Residential mortgage-backed securities ("RMBS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	903	1,778
Gross unrealized gains	53	82
Gross unrealized losses	(17)	(69)
Fair value	939	1,791
Commercial mortgage-backed securities ("CMBS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	776	1,425
Gross unrealized gains	47	60
Gross unrealized losses	(17)	(77)
Fair value	806	1,408
Redeemable preferred stock
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	15	15
Gross unrealized gains	2	3
Fair value	$ 17	$ 18