Investments (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 590,650	$ 658,802
Gross Unrealized Gains	18,104	19,277
Gross Unrealized Losses	(1,135)	(5,870)
Estimated Fair Value	607,619	672,209
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
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	589,927	658,079
Gross Unrealized Gains	18,104	19,277
Gross Unrealized Losses	(1,135)	(5,870)
Estimated Fair Value	606,896	671,486
Debt Securities \| U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	139,566	144,497
Gross Unrealized Gains	347	0
Gross Unrealized Losses	(106)	(2,818)
Estimated Fair Value	139,807	141,679
Debt Securities \| State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	152,914	157,603
Gross Unrealized Gains	9,066	9,453
Gross Unrealized Losses	0	(4)
Estimated Fair Value	161,980	167,052
Debt Securities \| Mortgage-Backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	296,369	354,631
Gross Unrealized Gains	8,691	9,824
Gross Unrealized Losses	(946)	(2,936)
Estimated Fair Value	304,114	361,519
Debt Securities \| Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,078	1,348
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(83)	(112)
Estimated Fair Value	$ 995	$ 1,236

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB

-Name Accounting Standards Codification

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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