Amortized Cost and Estimated Fair Values of Investments Available-for-sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 815,545	$ 825,582
Available-for-Sale Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	806,658	791,889
Gross Unrealized Gains	19,818	34,160
Gross Unrealized Losses	(10,931)	(467)
Estimated Fair Value	815,545	825,582
Available-for-Sale Securities | Marketable Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	723	723
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	723	723
Available-for-Sale Securities | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	805,935	791,166
Gross Unrealized Gains	19,818	34,160
Gross Unrealized Losses	(10,931)	(467)
Estimated Fair Value	814,822	824,859
Available-for-Sale Securities | Debt Securities | U.S. Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	177,979	155,442
Gross Unrealized Gains	126	1,084
Gross Unrealized Losses	(5,862)	(98)
Estimated Fair Value	172,243	156,428
Available-for-Sale Securities | Debt Securities | State and Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	159,772	160,496
Gross Unrealized Gains	7,164	13,996
Gross Unrealized Losses	(70)	(1)
Estimated Fair Value	166,866	174,491
Available-for-Sale Securities | Debt Securities | Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	464,483	471,527
Gross Unrealized Gains	12,523	19,080
Gross Unrealized Losses	(4,737)	(128)
Estimated Fair Value	472,269	490,479
Available-for-Sale Securities | Debt Securities | Corporate Debt
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,000	2,000
Gross Unrealized Gains	5	0
Gross Unrealized Losses	0	(4)
Estimated Fair Value	2,005	1,996
Available-for-Sale Securities | Debt Securities | Trust Preferred
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,701	1,701
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(262)	(236)
Estimated Fair Value	$ 1,439	$ 1,465