Investments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 security	Mar. 31, 2014	Dec. 31, 2014 security
Schedule Of Investments [Line Items]
Fair value of securities on deposit with various regulatory authorities	$ 10		$ 10
Fair value of securities pledged as collateral	530		532
Cash and money market securities pledged as collateral under investment agreements	$ 27		$ 26
Weighted average contractual maturity period in years for securities in an unrealized loss position	16 years	13 years
Number of securities in unrealized loss position for a continuous 12 month period	71		143
Rate that a security's fair value is below book value	5.00%
Fair Value Below Book Value Greater Than Five Percent [Member]
Schedule Of Investments [Line Items]
Number of securities in unrealized loss position for a continuous 12 month period	15		23