Investments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available for Sale Securities
Amortized cost	$ 58,129	$ 71,915
Gross unrealized gains	2,733	5,632
Gross unrealized losses	(567)	(530)
Fair value	60,295	77,017
U.S. government and agencies
Schedule of Available for Sale Securities
Amortized cost	2,725	4,387
Gross unrealized gains	158	326
Gross unrealized losses	(2)
Fair value	2,881	4,713
Municipal
Schedule of Available for Sale Securities
Amortized cost	9,246	12,139
Gross unrealized gains	451	1,038
Gross unrealized losses	(86)	(108)
Fair value	9,611	13,069
Corporate
Schedule of Available for Sale Securities
Amortized cost	38,285	44,943
Gross unrealized gains	1,777	3,721
Gross unrealized losses	(365)	(127)
Fair value	39,697	48,537
Foreign government
Schedule of Available for Sale Securities
Amortized cost	1,831	2,290
Gross unrealized gains	119	228
Gross unrealized losses	(11)	(1)
Fair value	1,939	2,517
Asset-backed securities ("ABS")
Schedule of Available for Sale Securities
Amortized cost	3,389	3,623
Gross unrealized gains	75	108
Gross unrealized losses	(43)	(107)
Fair value	3,421	3,624
Residential mortgage-backed securities ("RMBS")
Schedule of Available for Sale Securities
Amortized cost	1,787	3,000
Gross unrealized gains	100	142
Gross unrealized losses	(43)	(110)
Fair value	1,844	3,032
Commercial mortgage-backed securities ("CMBS")
Schedule of Available for Sale Securities
Amortized cost	844	1,510
Gross unrealized gains	48	65
Gross unrealized losses	(17)	(77)
Fair value	875	1,498
Redeemable preferred stock
Schedule of Available for Sale Securities
Amortized cost	22	23
Gross unrealized gains	5	4
Fair value	$ 27	$ 27