Amounts Reclassified from Each Component of Accumulated Other Comprehensive (Loss) Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on curtailment of nonqualified pension plan					$ 690
Income before income taxes	9,638		8,310		27,140		24,240
Less: income tax (expense) benefit	3,237		2,885		9,167		8,397
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realization of gain on sale of investment securities available for sale			(416)		(2)		(1,132)
Less: income tax (expense) benefit			(146)		(1)		(396)
Net of income tax			(270)		(1)		(736)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Unfunded pension liability
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss included in net periodic pension costs	463	[1]	488	[1]	1,390	[1]	1,466	[1]
Amortization of prior service cost included in net periodic pension costs	3	[1]	21	[1]	10	[1]	62	[1]
Amortization of transition obligation included in net periodic pension costs			7	[1]			20	[1]
Net gain on curtailment of nonqualified pension plan					(690)
Income before income taxes	466		516		710		1,548
Less: income tax (expense) benefit	163		181		249		703
Net of income tax	$ 303		$ 335		$ 461		$ 845

[1] Accumulated other comprehensive loss components are included in the computation of net periodic pension cost. See Note 10 - Pension and Other Post-Retirement Benefit Plans