Investments (details) - Derivatives (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Derivative
Net realized investment gain (loss)	$ 10	[1]	$ 10	[1]
Futures [Member] | U.S. Treasury notes contracts [Member]
Derivative
Description of derivative terms	The Company enters into 90-day futures contracts on U.S. Treasury notes which require a daily mark-to-market and settlement with the broker.				The Company enters into 90-day futures contracts on U.S. Treasury notes which require a daily mark-to-market and settlement with the broker.
Additional details of underlying contract	10-year Treasury notes				5-year Treasury notes
Notional value of open contracts	2,000				800
Net realized investment gain (loss)	$ (19)		$ 6

[1] Total other-than-temporary impairment (OTTI) gains were $0 million for each of the three months ended March 31, 2013 and 2012. Of total OTTI, credit losses of $(5) million and $(4) million for the three months ended March 31, 2013 and 2012, respectively, were recognized in net realized investment gains. In addition, unrealized gains from other changes in total OTTI of $5 million and $4 million for the three months ended March 31, 2013 and 2012, respectively, were recognized in other comprehensive income as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.