Statements of Condition (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets
Cash and cash equivalents	$ 86,406	$ 72,314
Investments available-for-sale (at fair value)	825,582	951,301
Other assets	114,833	111,388
Total assets	3,955,206	3,711,370	3,519,388
Liabilities
Subordinated debentures	35,000	35,000
Accrued expenses and other liabilities	31,673	24,720
Total liabilities	3,471,694	3,265,261
Stockholders' Equity
Common stock	24,905	24,091
Warrants	24,905	24,091
Additional paid in capital	191,689	177,828
Retained earnings	255,606	230,942
Accumulated other comprehensive loss	11,312	13,248	(2,620)	(2,652)
Total stockholders' equity	483,512	446,109	407,569	373,586
Total liabilities and stockholders' equity	3,955,206	3,711,370
Parent Company
Assets
Cash and cash equivalents	10,035	8,326
Investments available-for-sale (at fair value)	723	100
Investment in subsidiary	472,938	424,965
Loan to subsidiary	35,000	35,000
Dividend receivable from subsidiary	0	13,000
Other assets	338	327
Total assets	519,034	481,718
Liabilities
Subordinated debentures	35,000	35,000
Accrued expenses and other liabilities	522	609
Total liabilities	35,522	35,609
Stockholders' Equity
Common stock	24,905	24,091
Additional paid in capital	191,689	177,828
Retained earnings	255,606	230,942
Accumulated other comprehensive loss	11,312	13,248
Total stockholders' equity	483,512	446,109
Total liabilities and stockholders' equity	$ 519,034	$ 481,718