Investments (Amortized Cost and Estimated Fair Values of Investments Held-To-Maturity) (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	$ 216,866	$ 219,973
Gross Unrealized Gains	3,480	4,346
Gross Unrealized Losses	(2,466)	(2,059)
Investments held-to-maturity, fair value	217,880	222,260
U.S. Government Agencies
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	64,515	64,512
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,340)	(1,734)
Investments held-to-maturity, fair value	63,175	62,778
State and municipal
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	150,069	155,261
Gross Unrealized Gains	3,454	4,321
Gross Unrealized Losses	(1,126)	(325)
Investments held-to-maturity, fair value	152,397	159,257
Mortgage-Backed
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	182	200
Gross Unrealized Gains	26	25
Gross Unrealized Losses	0	0
Investments held-to-maturity, fair value	208	225
Corporate Debt
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	2,100	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Investments held-to-maturity, fair value	$ 2,100	$ 0