Other Comprehensive Income (Components of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,196,065	$ 1,144,099
Current period other comprehensive (loss) income	$ (1,144)	$ 3,561	8,520	2,069
Ending Balance	1,244,280	1,183,973	1,244,280	1,183,973
Unrealized gains on securities available for sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	13,967	6,139	3,951	7,743
Current period other comprehensive (loss) income	(1,515)	3,678	8,501	2,074
Ending Balance	12,452	9,817	12,452	9,817
Post-retirement obligations [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(6,185)	(7,602)	(6,424)	(7,714)
Current period other comprehensive (loss) income	141	115	380	227
Ending Balance	(6,044)	(7,487)	(6,044)	(7,487)
Unrealized losses on derivatives (cash flow hedges) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(664)	0	(73)	0
Current period other comprehensive (loss) income	230	(232)	(361)	(232)
Ending Balance	(434)	(232)	(434)	(232)
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	7,118	(1,463)	(2,546)	29
Current period other comprehensive (loss) income	(1,144)	3,561	8,520	2,069
Ending Balance	$ 5,974	$ 2,098	$ 5,974	$ 2,098