Parent Company Financial Information (Statements of Condition) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 96,217,000	$ 74,427,000
Investments available-for-sale (at fair value)	672,209,000	751,284,000
Other assets	117,282,000	115,912,000
Total assets	4,397,132,000	4,106,100,000	3,955,206,000
Liabilities
Subordinated debentures	35,000,000	35,000,000
Accrued expenses and other liabilities	44,440,000	25,670,000
Total liabilities	3,875,381,000	3,606,737,000
Stockholders Equity
Common stock	25,045,000	24,990,000
Additional paid in capital	194,647,000	193,445,000
Retained earnings	302,882,000	283,898,000
Accumulated other comprehensive income (loss)	(823,000)	(2,970,000)	11,312,000	13,248,000
Total stockholders equity	521,751,000	499,363,000	483,512,000	446,109,000
Total liabilities and stockholders equity	4,397,132,000	4,106,100,000
Parent Company
Assets
Cash and cash equivalents	12,088,000	11,551,000
Investments available-for-sale (at fair value)	723,000	723,000
Investment in subsidiary	508,821,000	487,231,000
Loan to subsidiary	35,000,000	35,000,000
Dividend receivable from subsidiary	0	0
Other assets	342,000	335,000
Total assets	556,974,000	534,840,000
Liabilities
Subordinated debentures	35,000,000	35,000,000
Accrued expenses and other liabilities	223,000	477,000
Total liabilities	35,223,000	35,477,000
Stockholders Equity
Common stock	25,045,000	24,990,000
Additional paid in capital	194,647,000	193,445,000
Retained earnings	302,882,000	283,898,000
Accumulated other comprehensive income (loss)	(823,000)	(2,970,000)
Total stockholders equity	521,751,000	499,363,000
Total liabilities and stockholders equity	$ 556,974,000	$ 534,840,000