Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 113,863	$ 141,814	$ 140,434
Other comprehensive income (loss) before reclassification	(133,481)	(32,453)	19,551
Amounts reclassified from accumulated other comprehensive income (loss)	(5,005)	4,502	(18,171)
Net other comprehensive income (loss) during period	(138,486)	(27,951)	1,380
Ending Balance	(24,623)	113,863	141,814
Securities Available For Sale [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	160,611	190,589	146,672
Other comprehensive income (loss) before reclassification	(134,725)	(29,933)	43,942
Amounts reclassified from accumulated other comprehensive income (loss)	(9,733)	(45)	(25)
Net other comprehensive income (loss) during period	(144,458)	(29,978)	43,917
Ending Balance	16,153	160,611	190,589
Defined Benefit Plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(46,748)	(48,775)	(26,131)
Other comprehensive income (loss) before reclassification	1,244	(2,520)	(24,391)
Amounts reclassified from accumulated other comprehensive income (loss)	4,728	4,547	1,747
Net other comprehensive income (loss) during period	5,972	2,027	(22,644)
Ending Balance	(40,776)	(46,748)	(48,775)
Derivatives [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	0	0	19,893
Other comprehensive income (loss) before reclassification	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	(19,893)
Net other comprehensive income (loss) during period	0	0	(19,893)
Ending Balance	$ 0	$ 0	$ 0