Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Parenthetical) (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Unrealized Gains (Losses) on Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) of related to sales of securities classified as available-for-sale	[1]	$ (2,055)	$ 14,729	$ (2,468)	$ 13,790

[1] For the three and six months ended June 30, 2018, net realized losses of $2.1 million and $2.5 million, respectively, related to sales of debt securities classified as available-for-sale. For the three and six months ended June 30, 2017, net realized gains of $14.7 million and $13.8 million, respectively, related to sales of debt and equity securities classified as available-for-sale.