Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ (24,623)	$ 113,863	$ 141,814
Other comprehensive income (loss) before reclassification	97,394	(133,481)	(32,453)
Amounts reclassified from accumulated other comprehensive income (loss)	6,741	(5,005)	4,502
Net other comprehensive income (loss) during period	104,135	(138,486)	(27,951)
Ending Balance	79,512	(24,623)	113,863
Securities Available For Sale [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	16,153	160,611	190,589
Other comprehensive income (loss) before reclassification	97,865	(134,725)	(29,933)
Amounts reclassified from accumulated other comprehensive income (loss)	3,212	(9,733)	(45)
Net other comprehensive income (loss) during period	101,077	(144,458)	(29,978)
Ending Balance	117,230	16,153	160,611
Defined Benefit Plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(40,776)	(46,748)	(48,775)
Other comprehensive income (loss) before reclassification	(471)	1,244	(2,520)
Amounts reclassified from accumulated other comprehensive income (loss)	3,529	4,728	4,547
Net other comprehensive income (loss) during period	3,058	5,972	2,027
Ending Balance	$ (37,718)	$ (40,776)	$ (46,748)

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
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