Comprehensive Income - Components of Other Comprehensive Income and Related Tax Effects (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Unrealized holding losses on available for sale securities before reclassifications	$ (11,015)	$ (1,895)	$ (162)
Amounts reclassified out of accumulated other comprehensive income	0	0	0
Unrealized holding losses on available for sale securities after reclassifications	(11,015)	(1,895)	(162)
Tax effect	4,631	797	68
Unrealized holding losses on available for sale securities, net of tax	(6,384)	(1,098)	(94)
Change in unfunded status of the supplemental retirement plans before reclassifications	511	1,384	(7,253)
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(40)	(57)	138
Amortization of actuarial losses	550	823	17
Total amounts reclassified out of accumulated other comprehensive income	510	766	155
Change in unfunded status of the supplemental retirement plans after reclassifications	1,021	2,150	(7,098)
Tax effect	(429)	(904)	2,984
Change in unfunded status of the supplemental retirement plans, net of tax	592	1,246	(4,114)
Change in joint beneficiary agreement liability before reclassifications	(343)	277	148
Amounts reclassified out of accumulated other comprehensive income	0	0	0
Change in joint beneficiary agreement liability after reclassifications	(343)	277	148
Tax effect	0	0	0
Change in joint beneficiary agreement liability, net of tax	(343)	277	148
Total other comprehensive income (loss)	$ (6,135)	$ 425	$ (4,060)