Investments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 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	638	693
Cash and money market securities pledged as collateral under investment agreements	$ 37	$ 22
Weighted average contractual maturity period in years for securities in an unrealized loss position	15 years	18 years
Number of Securities in unrealized loss position	191	77
Rate that a security's fair value is below book value	5.00%
Greater Than Five Percent [Member]
Schedule Of Investments [Line Items]
Number of Securities in unrealized loss position	32	50