Investments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 Y	Dec. 31, 2011 Y
Schedule of Investments [Line Items]
Fair value of securities on deposit with various regulatory authorities	$ 10	$ 11
Fair value of securities pledged as collateral	820	1,900
Cash pledged as collateral under investment agreements	$ 144	$ 224
Weighted average contractual maturity period in years for securities in an unrealized loss position	23	21
Number of securities, unrealized loss position for continuous twelve-month period or longer	153	290
Rate that a security's 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	89	218