Other Comprehensive Income - Schedule of Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity [Abstract]
Net gains (losses) arising during the period, Before Tax	$ (3,458)	$ 33,329	$ (22,679)
Less reclassification adjustment for net gains included in net income, Before Tax	(2,487)	(745)	(14,678)
Net change, Unrealized gains on AFS securities, Before Tax	(5,945)	32,584	(37,357)
Net change in non-credit impairment losses on securities, Before Tax	(416)	2,299	6,266
Less reclassification adjustment for credit impairment losses included in net income, Before Tax	(728)	(661)	(461)
Net change, Non-credit impairment losses on securities available for sale and held to maturity, Before Tax	(1,144)	1,638	5,805
Net gains (losses) gains arising during the period, Before Tax	(12,360)	(20,910)	4,255
Less reclassification adjustment for losses included in net income, Before Tax	7,075	6,663	6,898
Net change, Unrealized gains and losses on derivatives (cash flow hedges), Before Tax	(5,285)	(14,247)	11,153
Net gains (losses) arising during the period, Before Tax	6,030	(27,902)	37,204
Amortization of prior service cost, Before Tax	206	305	2,440
Amortization of net loss, Before Tax	790	226	1,970
Recognition of loss due to curtailment	0	0	750
Net change, Defined benefit pension plan, Before Tax	7,026	(27,371)	42,364
Total other comprehensive income (loss), Before Tax	(5,348)	(7,396)	21,965
Net gains (losses) arising during the period, Tax Effect	1,458	(13,931)	9,440
Less reclassification adjustment for net gains included in net income, Tax Effect	1,041	312	6,156
Net change in non-credit impairment losses on securities, Tax Effect	2,499	(13,619)	15,596
Net change in non-credit impairment losses on securities	175	(965)	(2,629)
Less reclassification adjustment for credit impairment losses included in net income, Tax Effect	304	278	193
Net change, Non-credit impairment losses on securities available for sale and held to maturity, Tax Effect	479	(687)	(2,436)
Net (losses) gains arising during the period, Tax Effect	5,121	8,763	(1,855)
Less reclassification adjustment for losses included in net income, Tax Effect	(2,948)	(2,777)	(2,893)
Net change, Unrealized gains and losses on derivatives (cash flow hedges), Tax Effect	2,173	5,986	(4,748)
Net gains (losses) arising during the period, Tax Effect	(2,586)	11,695	(15,627)
Amortization of prior service cost, Tax Effect	(89)	(128)	(986)
Amortization of net loss, Tax Effect	(328)	(94)	(825)
Recognition of loss due to curtailment	0	0	(282)
Net change, Defined benefit pension plan, Tax Effect	(3,003)	11,473	(17,720)
Total other comprehensive income (loss), Tax Effect	2,148	3,153	(9,308)
Net gains (losses) arising during the period, After Tax	(2,000)	19,398	(13,239)
Less reclassification adjustment for net gains included in net income, After Tax	(1,446)	(433)	(8,522)
Total	(3,446)	18,965	(21,761)
Net change in non-credit impairment losses on securities, After Tax	(241)	1,334	3,637
Less reclassification adjustment for credit impairment losses included in net income, After Tax	(424)	(383)	(268)
Total	(665)	951	3,369
Net gains (losses) arising during the period, After Tax	(7,239)	(12,147)	2,400
Less reclassification adjustment for losses included in net income, After Tax	4,127	3,886	4,005
Total	(3,112)	(8,261)	6,405
Net gains (losses) arising during the period, After Tax	3,444	(16,207)	21,577
Amortization of prior service cost, After Tax	117	177	1,454
Amortization of net loss, After Tax	462	132	1,145
Recognition of loss due to curtailment	0	0	468
Total	4,023	(15,898)	24,644
Total other comprehensive (loss) income	$ (3,200)	$ (4,243)	$ 12,657