Investment Securities (Investment Securities Held to Maturity) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 472,934	$ 473,684
Gross unrealized gains	18,672	15,175
Gross unrealized losses	(257)	(528)
Fair value	491,349	488,331
Agency Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	4,197	4,096
Gross unrealized gains	8	9
Gross unrealized losses	0	(8)
Fair value	4,205	4,097
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,403	1,597
Gross unrealized gains	55	61
Gross unrealized losses	0	0
Fair value	1,458	1,658
State And Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	457,427	458,062
Gross unrealized gains	18,557	15,094
Gross unrealized losses	(251)	(495)
Fair value	475,733	472,661
Corporate Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	9,907	9,929
Gross unrealized gains	52	11
Gross unrealized losses	(6)	(25)
Fair value	$ 9,953	$ 9,915