Investments and Derivative Instruments (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Other-Than-Temporary Impairment Losses
Balance as of beginning of period	$ (1,676)	$ (2,072)
Additions for credit impairments recognized on securities not previously impaired	(12)	(28)
Additions for credit impairments recognized on securities previously impaired	(5)	(17)
Securities that matured or were sold during the period	160	109
Securities due to an increase in expected cash flows	3	5
Balance as of end of period	$ (1,530)	$ (2,003)