Investments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 security	Dec. 31, 2013 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	532	735
Cash and money market securities pledged as collateral under investment agreements	$ 26	$ 22
Weighted average contractual maturity period in years for securities in an unrealized loss position	13 years	18 years
Number of securities in unrealized loss position for a continuous 12 month period	143	77
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	23	50