Fair Value Measurements (details) - Changes in Level 3 (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	$ 284	$ 294
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	1	9
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	12	7
Level 3 fair value category purchases	42	82
Level 3 fair value category sales	(16)
Level 3 fair value category settlements/maturities	(20)	(94)
Gross transfers into Level 3		10
Gross transfers out of Level 3		(24)
Level 3 fair value category, ending balance	303	284
Fixed maturities [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	230	250
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	1	4
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	1	5
Level 3 fair value category purchases	42	79
Level 3 fair value category settlements/maturities	(20)	(94)
Gross transfers into Level 3		10
Gross transfers out of Level 3		(24)
Level 3 fair value category, ending balance	254	230
Other investments [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	54	44
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains		5
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	11	2
Level 3 fair value category purchases		3
Level 3 fair value category sales	(16)
Level 3 fair value category, ending balance	$ 49	$ 54