Investments (Amortized Cost and Estimated Fair Values of Investments Held-To-Maturity) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	$ 216,642	$ 219,973
Gross Unrealized Gains	4,314	4,346
Gross Unrealized Losses	(733)	(2,059)
Investments held-to-maturity, fair value	220,223	222,260
U.S. Government Agencies
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	64,517	64,512
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(410)	(1,734)
Investments held-to-maturity, fair value	64,113	62,778
State and municipal
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	149,849	155,261
Gross Unrealized Gains	4,283	4,321
Gross Unrealized Losses	(323)	(325)
Investments held-to-maturity, fair value	153,809	159,257
Mortgage-Backed
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	176	200
Gross Unrealized Gains	25	25
Gross Unrealized Losses	0	0
Investments held-to-maturity, fair value	201	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