Related Party (Affiliated Asset Transfers) (Details) (USD $)	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2011
Related Party Transactions [Line Items]
Fixed maturity asset transfer amortized cost	$ 90,000,000	$ 123,000,000		$ 27,000,000	$ 143,000,000
Fixed maturity asset transfer fair value	103,000,000	141,000,000		28,000,000	151,000,000
Realized gain on fixed maturity asset transfer	8,000,000	11,000,000		1,000,000	8,000,000
Commercial mortgage loans asset transfer amortized cost			6,000,000	27,000,000	49,000,000
Commercial mortgage loans asset transfer fair value			6,000,000	28,000,000	54,000,000
Realized gain on commerical mortgage loan asset transfer			$ 8,000	$ 1,000,000	$ 5,000,000