Fair Value Measurements (details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Mar. 31, 2012
Fair Value Measurements disclosure
Percent of fixed maturities for which a pricing service estimates fair value		98.00%
Fair value of the fixed maturities for which the Company used an internal pricing matrix	$ 88	$ 97
Fair value of the fixed maturities for which the Company received a broker quote	162	153
Convertible bonds held containing embedded conversion options that are valued separately from the host bond contract and disclosed in Level 2	22	23
Municipal fixed maturity securities valued using observable market data which resulted in a transfer out of Level 3 into Level 2	$ 81
Percent of short-term securities for which a pricing service estimates fair value	94.00%	93.00%
Percent of debt, other than commercial paper, for which a pricing service estimates fair value	93.00%	96.00%