Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on available for sale securities, Before tax amount	$ 1,477	$ 3,684	$ 4,769	$ 8,270
Unrealized gains (losses) on available for sale securities, Tax expense (benefit)	643	(619)	(184)	(527)
Amortization of prior service cost included in net periodic pension cost, Before tax amount	37	12	96	21
Direct Cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost included in net periodic pension cost, Before tax amount	37		96	21
Amortization of prior service cost included in net periodic pension cost, Tax expense (benefit)	(13)	(4)	(26)	(7)
Amortization of prior service cost included in net periodic pension cost, Net of tax	24	(4)	70	14
Other Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on available for sale securities, Before tax amount	(3,341)	(840)	(4,360)	(3,207)
Unrealized gains (losses) on available for sale securities, Tax expense (benefit)	1,154	213	1,498	936
Unrealized gains (losses) on available for sale securities, Net of tax	$ (2,187)	$ (627)	$ (2,862)	$ (2,271)