Other Comprehensive Income - Summary of Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Net change in fair value recorded in accumulated OCI, Before-tax	$ (10,235)	$ 2,083	$ (47,346)	$ 6,128
Net change in fair value recorded in accumulated OCI, Tax effect	3,025	(875)	13,997	(2,574)
Net change in fair value recorded in accumulated OCI, After-tax	(7,210)	1,208	(33,349)	3,554
Amortization of unrealized gains (losses) on securities transferred from available-for-sale to held-to-maturity, Before-tax	(152)	(862)	(1,809)	(2,841)
Amortization of unrealized gains (losses) on securities transferred from available-for-sale to held-to-maturity, Tax effect	45	362	535	1,193
Amortization of unrealized gains (losses) on securities transferred from available-for-sale to held-to-maturity, After-tax	(107)	(500)	(1,274)	(1,648)
Net realized gain reclassified into earnings, Before-tax	0	0	0	(402)
Net realized gain reclassified into earnings, Tax Effect	0	0	0	169
Net realized gain reclassified into earnings, After-Tax	0	0	0	(233)
Net change, Before-tax	(10,387)	1,221	(49,155)	2,885
Net change, Tax effect	(3,070)	513	(14,532)	1,212
Other comprehensive income (loss), net of tax	$ (7,317)	$ 708	$ (34,623)	$ 1,673