Parent Company Financial Information (Statements of Condition) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 72,882	$ 96,217
Investments available-for-sale (at fair value)	592,049	672,209
Investments held-to-maturity	208,265	219,973
Other assets	117,208	117,282
Total assets	4,655,380	4,397,132	$ 4,106,100
Liabilities
Subordinated debentures	35,000	35,000
Accrued expenses and other liabilities	38,078	44,440
Total liabilities	4,130,953	3,875,381
Stockholders Equity
Common stock	24,296	25,045
Additional paid in capital	175,588	194,647
Retained earnings	325,840	302,882
Accumulated other comprehensive income (loss)	(1,297)	(823)	(2,970)	$ 11,312
Total stockholders equity	524,427	521,751	$ 499,363	$ 483,512
Total liabilities and stockholders equity	4,655,380	4,397,132
Parent Company [Member]
Assets
Cash and cash equivalents	9,154	12,088
Investments available-for-sale (at fair value)	1,223	723
Investments held-to-maturity	2,100	0
Investment in subsidiary	511,841	508,821
Loan to subsidiary	35,000	35,000
Dividend receivable from subsidiary	0	0
Other assets	352	342
Total assets	559,670	556,974
Liabilities
Subordinated debentures	35,000	35,000
Accrued expenses and other liabilities	243	223
Total liabilities	35,243	35,223
Stockholders Equity
Common stock	24,296	25,045
Additional paid in capital	175,588	194,647
Retained earnings	325,840	302,882
Accumulated other comprehensive income (loss)	(1,297)	(823)
Total stockholders equity	524,427	521,751
Total liabilities and stockholders equity	$ 559,670	$ 556,974