Amortized Cost and Approximate Fair Values of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Amortized Cost	$ 531,600	$ 503,479
Gross Unrealized Gains	21,769	20,600
Gross Unrealized Losses	5,818	5,588
Fair Value	547,551	518,491
Amortized Cost	396,770	427,909
Gross Unrealized Gains	16,478	14,560
Gross Unrealized Losses	1
Fair Value	413,247	442,469
Amortized Cost	928,370	931,388
Gross Unrealized Gains	38,247	35,160
Gross Unrealized Losses	5,819	5,588
Fair Value	960,798	960,960
U.S. Government-sponsored agency securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	4,841	99
Gross Unrealized Gains	92	18
Fair Value	4,933	117
State and municipal
Gain (Loss) on Investments [Line Items]
Amortized Cost	136,011	136,857
Gross Unrealized Gains	9,614	10,496
Gross Unrealized Losses	28
Fair Value	145,597	147,353
Amortized Cost	119,222	120,171
Gross Unrealized Gains	4,250	3,785
Gross Unrealized Losses	1
Fair Value	123,471	123,956
U.S. Government-sponsored mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	371,971	358,928
Gross Unrealized Gains	11,644	10,086
Gross Unrealized Losses	53	16
Fair Value	383,562	368,998
Amortized Cost	277,548	307,738
Gross Unrealized Gains	12,228	10,775
Fair Value	289,776	318,513
Corporate obligations
Gain (Loss) on Investments [Line Items]
Amortized Cost	16,947	5,765
Gross Unrealized Gains	419
Gross Unrealized Losses	5,737	5,572
Fair Value	11,629	193
Equity Securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	1,830	1,830
Fair Value	$ 1,830	$ 1,830