Fair Value Measurements (details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair Value Measurements disclosure
Percent of fixed maturities for which a pricing service estimates fair value	98.00%
Convertible bonds held containing embedded conversion options that are valued separately from the host bond contract and disclosed in Level 2	$ 22,000,000	$ 37,000,000
Municipal fixed maturity securities valued using observable market data which resulted in a transfer out of Level 3 into Level 2	81,000,000
Carrying value of financial instruments classified as other assets (which approximates fair value)	552,000,000	647,000,000
Carrying value of financial instruments classified as other liabilities (which approximates fair value)	3,930,000,000	3,750,000,000
Debt, carrying value	6,605,000,000	6,611,000,000
Debt, fair value	$ 7,710,000,000	$ 7,210,000,000
Percent of debt, other than commercial paper, for which a pricing service estimates fair value	93.00%	94.00%