Fair Value of Assets and Liabilities (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
ARS backed by student loans | Discounted cash flow model
Quantitative information about the significant unobservable inputs
Fair value	213	394
ARS backed by student loans | Minimum | Discounted cash flow model
Quantitative information about the significant unobservable inputs
Anticipated date liquidity will return to the market	18 months	18 months
ARS backed by student loans | Minimum | Weighted average | Discounted cash flow model
Quantitative information about the significant unobservable inputs
Anticipated date liquidity will return to the market	31 months	31 months
ARS backed by student loans | Maximum | Discounted cash flow model
Quantitative information about the significant unobservable inputs
Anticipated date liquidity will return to the market	60 months	60 months
ARS backed by student loans | Maximum | Weighted average | Discounted cash flow model
Quantitative information about the significant unobservable inputs
Anticipated date liquidity will return to the market	43 months	43 months
Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Fair value	(459)	(419)
Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Weighted average | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	1.89%	1.92%
Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Minimum | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	1.00%	1.00%
Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Maximum | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	2.00%	2.00%
Fixed income securities - non-binding broker quotes
Quantitative information about the significant unobservable inputs
Fair value	1,740	1,870
Municipal - not rated by third party credit rating agencies
Quantitative information about the significant unobservable inputs
Fair value	303	395