Investment Securities Held to Maturity - Investment Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 473,684	$ 469,528
Gross unrealized gains	15,175	13,983
Gross unrealized losses	(528)	(1,038)
Fair value	488,331	482,473
Agency Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	4,096	6,813
Gross unrealized gains	9	17
Gross unrealized losses	(8)	(20)
Fair value	4,097	6,810
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,597	2,816
Gross unrealized gains	61	123
Gross unrealized losses	0	0
Fair value	1,658	2,939
State and Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	458,062	449,410
Gross unrealized gains	15,094	13,814
Gross unrealized losses	(495)	(986)
Fair value	472,661	462,238
Corporate Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	9,929	10,489
Gross unrealized gains	11	29
Gross unrealized losses	(25)	(32)
Fair value	$ 9,915	$ 10,486