Investments (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 612,578	$ 658,802
Gross Unrealized Gains	17,101	19,277
Gross Unrealized Losses	(3,860)	(5,870)
Estimated Fair Value	625,819	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	611,855	658,079
Gross Unrealized Gains	17,101	19,277
Gross Unrealized Losses	(3,860)	(5,870)
Estimated Fair Value	625,096	671,486
Debt Securities | U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	139,542	144,497
Gross Unrealized Gains	122	0
Gross Unrealized Losses	(1,684)	(2,818)
Estimated Fair Value	137,980	141,679
Debt Securities | State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	153,769	157,603
Gross Unrealized Gains	8,185	9,453
Gross Unrealized Losses	(1)	(4)
Estimated Fair Value	161,953	167,052
Debt Securities | Mortgage-Backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	317,433	354,631
Gross Unrealized Gains	8,794	9,824
Gross Unrealized Losses	(2,082)	(2,936)
Estimated Fair Value	324,145	361,519
Debt Securities | Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,111	1,348
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(93)	(112)
Estimated Fair Value	$ 1,018	$ 1,236