Comprehensive Income - Components of Other Comprehensive Income and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Equity [Abstract]
Unrealized holding (losses) gains on available for sale securities before reclassifications	$ 7,173	$ (4,752)	$ 13,298	$ (4,737)
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Unrealized holding (losses) gains on available for sale securities after reclassifications	7,173	(4,752)	13,298	(4,737)
Tax effect	(3,016)	1,998	(5,591)	1,992
Unrealized holding (losses) gains on available for sale securities, net of tax	4,157	(2,754)	7,707	(2,745)
Change in unfunded status of the supplemental retirement plans before reclassifications	0	0	0	0
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(20)	(14)	(20)	(28)
Amortization of actuarial losses	275	206	275	412
Total amounts reclassified out of accumulated other comprehensive income	255	192	255	384
Change in unfunded status of the supplemental retirement plans after reclassifications	255	192	255	384
Tax effect	(107)	(81)	(107)	(162)
Change in unfunded status of the supplemental retirement plans, net of tax	148	111	148	222
Change in joint beneficiary agreement liability before reclassifications	(4)		(4)
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Change in joint beneficiary agreement liability after reclassifications	(4)		(4)
Tax effect	0	0	0	0
Change in joint beneficiary agreement liability, net of tax	(4)		(4)
Total other comprehensive income (loss)	$ 4,301	$ (2,643)	$ 7,851	$ (2,523)