Investments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 Y	Dec. 31, 2012 Y
Schedule of Investments [Line Items]
Fair value of securities on deposit with various regulatory authorities	$ 11	$ 10
Fair value of securities pledged as collateral	804	820
Cash pledged as collateral under investment agreements	$ 122	$ 144
Weighted average contractual maturity period in years for securities in an unrealized loss position	24	23
Number of securities, unrealized loss position for continuous twelve-month period or longer	124	153
Rate that a securities fair value is below book value	5.00%
Fair Value Below Book Value By More Than 5% [Member]
Schedule of Investments [Line Items]
Number of securities, unrealized loss position for continuous twelve-month period or longer	68	89