Fair Value Measurements (details) - Changes in Level 3 (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	$ 303	$ 284	$ 294
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	14	15	9
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	(15)	(3)	7
Level 3 fair value category purchases	35	77	82
Level 3 fair value category sales	(41)	(57)
Level 3 fair value category settlements/maturities	(27)	(47)	(94)
Gross transfers into Level 3	13	13	10
Gross transfers out of Level 3	(24)	(24)	(24)
Level 3 fair value category, ending balance	258	258	284
Fixed maturities [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	254	230	250
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	1	2	4
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	(3)	(2)	5
Level 3 fair value category purchases	35	77	79
Level 3 fair value category sales	(24)	(24)
Level 3 fair value category settlements/maturities	(27)	(47)	(94)
Gross transfers into Level 3	13	13	10
Gross transfers out of Level 3	(24)	(24)	(24)
Level 3 fair value category, ending balance	225	225	230
Other investments [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	49	54	44
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	13	13	5
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income	(12)	(1)	2
Level 3 fair value category purchases			3
Level 3 fair value category sales	(17)	(33)
Level 3 fair value category, ending balance	$ 33	$ 33	$ 54