Other Comprehensive Income - Schedule of Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Net gains (losses) arising during the period, Before Tax	$ 33,329	$ (22,679)	$ 9,993
Less reclassification adjustment for net gains included in net income, Before Tax	(745)	(14,678)	(2,587)
Net change, Unrealized gains on AFS securities, Before Tax	32,584	(37,357)	7,406
Net change in non-credit impairment losses on securities, Before Tax	2,299	6,266	15,032
Less reclassification adjustment for credit impairment losses included in net income, Before Tax	(661)	(461)	4,220
Net change, Non-credit impairment losses on securities available for sale and held to maturity, Before Tax	1,638	5,805	19,252
Net gains (losses) gains arising during the period, Before Tax	(20,910)	4,255	(5,773)
Less reclassification adjustment for losses included in net income, Before Tax	6,663	6,898	6,478
Net change, Unrealized gains and losses on derivatives (cash flow hedges), Before Tax	(14,247)	11,153	705
Net gains (losses) arising during the period, Before Tax	(27,902)	37,204	(11,747)
Amortization of prior service cost, Before Tax	305	2,440	105
Amortization of net loss, Before Tax	226	1,970	2,401
Recognition of loss due to curtailment	0	750	0
Net change, Defined benefit pension plan, Before Tax	(27,371)	42,364	(9,241)
Total other comprehensive income (loss), Before Tax	(7,396)	21,965	18,122
Net gains (losses) arising during the period, Tax Effect	(13,931)	9,440	(4,311)
Less reclassification adjustment for net gains included in net income, Tax Effect	312	6,156	1,042
Net change in non-credit impairment losses on securities, Tax Effect	(13,619)	15,596	(3,269)
Net change in non-credit impairment losses on securities	(965)	(2,629)	(5,090)
Less reclassification adjustment for credit impairment losses included in net income, Tax Effect	278	193	(1,771)
Net change, Non-credit impairment losses on securities available for sale and held to maturity, Tax Effect	(687)	(2,436)	(6,861)
Net (losses) gains arising during the period, Tax Effect	8,763	(1,855)	2,422
Less reclassification adjustment for losses included in net income, Tax Effect	(2,777)	(2,893)	(2,718)
Net change, Unrealized gains and losses on derivatives (cash flow hedges), Tax Effect	5,986	(4,748)	(296)
Net gains (losses) arising during the period, Tax Effect	11,695	(15,627)	4,908
Amortization of prior service cost, Tax Effect	(128)	(986)	(67)
Amortization of net loss, Tax Effect	(94)	(825)	(1,005)
Recognition of loss due to curtailment	0	(282)	0
Net change, Defined benefit pension plan, Tax Effect	11,473	(17,720)	3,836
Total other comprehensive income (loss), Tax Effect	3,153	(9,308)	(6,590)
Net gains (losses) arising during the period, After Tax	19,398	(13,239)	5,682
Less reclassification adjustment for net gains included in net income, After Tax	(433)	(8,522)	(1,545)
Total	18,965	(21,761)	4,137
Net change in non-credit impairment losses on securities, After Tax	1,334	3,637	9,942
Less reclassification adjustment for credit impairment losses included in net income, After Tax	(383)	(268)	2,449
Total	951	3,369	12,391
Net gains (losses) arising during the period, After Tax	(12,147)	2,400	(3,351)
Less reclassification adjustment for losses included in net income, After Tax	3,886	4,005	3,760
Total	(8,261)	6,405	409
Net gains (losses) arising during the period, After Tax	(16,207)	21,577	(6,839)
Amortization of prior service cost, After Tax	177	1,454	38
Amortization of net loss, After Tax	132	1,145	1,396
Recognition of loss due to curtailment	0	468	0
Total	(15,898)	24,644	(5,405)
Total other comprehensive (loss) gain	$ (4,243)	$ 12,657	$ 11,532