Investment Securities - Schedule of Proceeds from Sales or Calls and Realized Gain and Losses of Available-for-sale Investment Securities and Other Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Proceeds from sales of available-for-sale securities	$ 47,064	$ 152,689	$ 131,321	$ 186,277
Proceeds from calls of available-for-sale securities			220,753	252,486
Total	58,275	213,289	159,968	257,397
Realized gains on sales of available-for-sale securities	1,251	2,954	3,259	4,283
Realized losses on sales of available-for-sale securities	(48)	(13)	(1,305)	(43)
Other securities gains (losses)	12	141	97	343
Net securities gains (losses)	1,494	3,075	2,282	4,575
Calls [Member]
Debt Securities, Available-for-sale [Line Items]
Proceeds from calls of available-for-sale securities	11,211	60,600	28,647	71,120
Realized gains on calls of available-for-sale securities	283		284
Realized losses on calls of available-for-sale securities	$ (4)	$ (7)	$ (53)	$ (8)