Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Amortized Cost	$ 498,104	$ 503,479
Gross Unrealized Gains	21,184	20,600
Gross Unrealized losses	5,945	5,588
Fair Value	513,343	518,491
Amortized Cost	361,020	427,909
Gross Unrealized Gains	17,170	14,560
Gross Unrealized Losses	16
Fair Value	378,174	442,469
Amortized Cost	859,124	931,388
Gross Unrealized Gains	38,354	35,160
Gross Unrealized Losses	5,961	5,588
Fair Value	891,517	960,960
Government Agency and Municipal Bonds and Notes
Gain (Loss) on Investments [Line Items]
Amortized Cost	4,475	99
Gross Unrealized Gains	165	18
Gross Unrealized losses
Fair Value	4,640	117
State and municipal
Gain (Loss) on Investments [Line Items]
Amortized Cost	148,187	136,857
Gross Unrealized Gains	10,025	10,496
Gross Unrealized losses	18
Fair Value	158,194	147,353
Amortized Cost	117,227	120,171
Gross Unrealized Gains	5,489	3,785
Gross Unrealized Losses	1
Fair Value	122,715	123,956
U.S. Government-sponsored mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	337,631	358,928
Gross Unrealized Gains	10,994	10,086
Gross Unrealized losses	46	16
Fair Value	348,579	368,998
Amortized Cost	243,793	307,738
Gross Unrealized Gains	11,681	10,775
Gross Unrealized Losses	15
Fair Value	255,459	318,513
Corporate Bonds and Notes
Gain (Loss) on Investments [Line Items]
Amortized Cost	6,105	5,765
Gross Unrealized Gains
Gross Unrealized losses	5,881	5,572
Fair Value	224	193
Equity securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	1,706	1,830
Gross Unrealized Gains
Gross Unrealized losses
Fair Value	$ 1,706	$ 1,830