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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of available-for-sale securities	$ 98,038	$ 50,368	$ 284,315	$ 157,819
Proceeds from calls of available-for-sale securities			337,766	1,043,014
Total	100,341	211,173	357,738	682,933
Realized gains on sales of available-for-sale securities	2,891	1,062	7,174	4,213
Realized losses on sales of available-for-sale securities	(36)	(2)	(79)	(450)
Other securities gains (losses)	104	125	447	101
Net securities gains (losses)	2,972	1,647	7,547	4,609
Calls [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from calls of available-for-sale securities	2,303	160,805	73,423	525,114
Realized gains on calls of available-for-sale securities	$ 13	477	13	848
Realized losses on calls of available-for-sale securities		$ (15)	$ (8)	$ (103)