Investments (Narrative) (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Schedule Of Investments [Line Items]
Fair value of securities on deposit with various regulatory authorities	$ 10	$ 11
Fair value of securities pledged as collateral	380	416
Cash and money market securities pledged as collateral under investment agreements	$ 1	$ 6
Weighted average contractual maturity period in years for securities in an unrealized loss position	15 years	22 years
Number of securities in unrealized loss position for a continuous 12 month period | security	59	46
Rate that a security's fair value is below book value	5.00%	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 | security	27	12