Comprehensive Income (Loss) - Components of Other Comprehensive Income and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity [Abstract]
Unrealized holding (losses) gains on available for sale securities before reclassifications	$ (2,058)	$ 3,997	$ 11,240	$ (740)
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Unrealized holding gains (losses) on available for sale securities after reclassifications	(2,058)	3,997	11,240	(740)
Tax effect	865	(1,681)	(4,726)	311
Unrealized holding gains (losses) on available for sale securities, net of tax	(1,193)	2,316	6,514	(429)
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	(10)	(14)	(30)	(42)
Amortization of actuarial losses	138	206	413	618
Total amounts reclassified out of accumulated other comprehensive income	128	192	383	576
Change in unfunded status of the supplemental retirement plans after reclassifications	128	192	383	576
Tax effect	(54)	(80)	(161)	(242)
Change in unfunded status of the supplemental retirement plans, net of tax	74	112	222	334
Change in joint beneficiary agreement liability before reclassifications	(1)		(5)
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Change in joint beneficiary agreement liability after reclassifications	(1)		(5)
Tax effect	0	0	0	0
Change in joint beneficiary agreement liability, net of tax	(1)		(5)
Total other comprehensive income (loss)	$ (1,120)	$ 2,428	$ 6,731	$ (95)