Other Comprehensive Income (Components of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,251,781	$ 1,196,065
Current period other comprehensive income	858	6,715
Ending Balance	1,267,092	1,214,122
Unrealized gains on securities available for sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(510)	3,951
Current period other comprehensive income	771	7,037
Ending Balance	261	10,988
Post-retirement obligations [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3,056)	(6,424)
Current period other comprehensive income	32	99
Ending Balance	(3,024)	(6,325)
Unrealized losses on derivatives (cash flow hedges) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	169	(73)
Current period other comprehensive income	55	(421)
Ending Balance	224	(494)
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3,397)	(2,546)
Current period other comprehensive income	858	6,715
Ending Balance	$ (2,539)	$ 4,169