PARENT COMPANY FINANCIAL INFORMATION (Statements of Condition) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 112,500	$ 134,125
Investments available-for-sale (at fair value)	729,507	733,554
Investments held-to-maturity	0	0
Other assets	121,469	124,137
Total assets	5,446,675	5,091,383	$ 4,655,380
Liabilities
Subordinated debentures	0	30,000
Accrued expenses and other liabilities	34,005	35,148
Total liabilities	4,882,859	4,557,811
Stockholders Equity
Common stock	23,996	23,901
Additional paid in capital	168,188	165,871
Retained earnings	378,489	350,414
Accumulated other comprehensive income (loss)	(6,857)	(6,614)	(1,297)	$ (823)
Total stockholders equity	563,816	533,572	$ 524,427	$ 521,751
Total liabilities and stockholders equity	5,446,675	5,091,383
Parent Company [Member]
Assets
Cash and cash equivalents	13,237	10,869
Investments available-for-sale (at fair value)	9,644	10,357
Investment in subsidiary	541,062	513,083
Loan to subsidiary	0	30,000
Other assets	304	515
Total assets	564,247	564,824
Liabilities
Subordinated debentures	0	30,000
Accrued expenses and other liabilities	431	1,252
Total liabilities	431	31,252
Stockholders Equity
Common stock	23,996	23,901
Additional paid in capital	168,188	165,871
Retained earnings	378,489	350,414
Accumulated other comprehensive income (loss)	(6,857)	(6,614)
Total stockholders equity	563,816	533,572
Total liabilities and stockholders equity	$ 564,247	$ 564,824