Other Comprehensive Income - Schedule of Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before reclassification, net of tax	$ (1,296)	$ (3,039)	$ (6,036)
Reclassification, net of tax	5,311	6,641	2,836
Net change before tax	7,611	6,557	(5,348)
Net change, tax	(3,596)	(2,955)	2,148
Other comprehensive income (loss), net	4,015	3,602	(3,200)
Unrealized Gains and Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before reclassification, before tax	636	(7,294)	(3,458)
Before reclassification, tax	(284)	3,001	1,458
Before reclassification, net of tax	352	(4,293)	(2,000)
Reclassification adjustment before tax	20	(777)	(2,487)
Reclassification, tax	(9)	312	1,041
Reclassification, net of tax	11	(465)	(1,446)
Net change before tax	656	(8,071)	(5,945)
Net change, tax	(293)	3,313	2,499
Other comprehensive income (loss), net	363	(4,758)	(3,446)
Non-credit Impairment Losses on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before reclassification, before tax	849	719	(416)
Before reclassification, tax	(351)	(302)	175
Before reclassification, net of tax	498	417	(241)
Reclassification adjustment before tax	(284)	(921)	(728)
Reclassification, tax	117	382	304
Reclassification, net of tax	(167)	(539)	(424)
Net change before tax	565	(202)	(1,144)
Net change, tax	(234)	80	479
Other comprehensive income (loss), net	331	(122)	(665)
Unrealized Gains and Losses on Derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before reclassification, before tax	1,005	(4,035)	(12,360)
Before reclassification, tax	(429)	1,574	5,121
Before reclassification, net of tax	576	(2,461)	(7,239)
Reclassification adjustment before tax	8,579	13,034	7,075
Reclassification, tax	(3,551)	(5,393)	(2,948)
Reclassification, net of tax	5,028	7,641	4,127
Net change before tax	9,584	8,999	(5,285)
Net change, tax	(3,980)	(3,819)	2,173
Other comprehensive income (loss), net	5,604	5,180	(3,112)
Net (losses) gains arising during the period
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before reclassification, before tax	3,843	(5,837)	(6,030)
Before reclassification, tax	(1,121)	2,539	2,586
Before reclassification, net of tax	2,722	(3,298)	(3,444)
Amortization of prior service credit (cost )
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification adjustment before tax	(268)	300	(206)
Reclassification, tax	77	(119)	89
Reclassification, net of tax	(191)	181	(117)
Amortization of net loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification adjustment before tax	381	294	790
Reclassification, tax	(133)	(109)	(328)
Reclassification, net of tax	248	185	462
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before reclassification, net of tax	(2,722)	3,298	3,444
Reclassification, net of tax	439	4	579
Net change before tax	(3,194)	5,831	7,026
Net change, tax	911	(2,529)	(3,003)
Other comprehensive income (loss), net	$ (2,283)	$ 3,302	$ 4,023