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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of available-for-sale securities	$ 26,319	$ 59,772	$ 196,584	$ 76,295
Proceeds from calls of available-for-sale securities			401,579	178,156
Total	187,727	60,937	409,586	100,835
Realized gains on sales of available-for-sale securities	316	1,691	2,797	2,349
Realized losses on sales of available-for-sale securities	(22)	(37)	(47)	(37)
Other securities gains	18	35	80	203
Net securities gains	512	1,689	3,195	2,248
Calls [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from calls of available-for-sale securities	161,408	$ 1,165	213,002	24,540
Realized gains on calls of available-for-sale securities	212		380
Realized losses on calls of available-for-sale securities	$ (12)		$ (15)	$ (267)