Fair Value Measurements (details) - Changes in Level 3 (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	$ 258	$ 284	$ 294
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains		15	9
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	2	(1)	7
Level 3 fair value category purchases	42	119	82
Level 3 fair value category sales	(1)	(58)
Level 3 fair value category settlements/maturities	(11)	(58)	(94)
Gross transfers into Level 3	2	15	10
Gross transfers out of Level 3	(7)	(31)	(24)
Level 3 fair value category, ending balance	285	285	284
Fixed maturities [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	225	230	250
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	1	3	4
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	2		5
Level 3 fair value category purchases	42	119	79
Level 3 fair value category sales	(1)	(25)
Level 3 fair value category settlements/maturities	(11)	(58)	(94)
Gross transfers into Level 3	2	15	10
Gross transfers out of Level 3	(7)	(31)	(24)
Level 3 fair value category, ending balance	253	253	230
Other investments [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	33	54	44
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	(1)	12	5
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income		(1)	2
Level 3 fair value category purchases			3
Level 3 fair value category sales		(33)
Level 3 fair value category, ending balance	$ 32	$ 32	$ 54