Investments (Narrative) (Detail) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) 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 | $	$ 328	$ 353
Weighted average contractual maturity period in years for securities in an unrealized loss position	13 years	12 years
Number of securities in unrealized loss position for a continuous 12 month period | security	103	133
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	50	24