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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of available-for-sale securities	$ 342,233	$ 243,312	$ 343,486
Proceeds from calls of available-for-sale securities	438,818	1,177,130	764,649
Total	430,466	878,936	747,763
Realized gains on sales of available-for-sale securities	8,710	5,423	5,640
Realized losses on sales of available-for-sale securities	(263)	(450)	(518)
Other securities gains (losses)	667	100	6
Net securities gains	9,135	5,848	5,718
Calls [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from calls of available-for-sale securities	88,233	635,624	404,277
Realized gains on calls of available-for-sale securities	29	922	605
Realized losses on calls of available-for-sale securities	$ (8)	$ (147)	$ (15)