Other Comprehensive Income (Components of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,196,065	$ 1,144,099
Current - period other comprehensive income	6,715	3,706
Ending Balance	1,214,122	1,157,671
Unrealized gains on securities available for sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	3,951	7,743
Current - period other comprehensive income	7,037	3,710
Ending Balance	10,988	11,453
Post-retirement obligations [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(6,424)	(7,714)
Current - period other comprehensive income	99	(4)
Ending Balance	(6,325)	(7,718)
Unrealized losses on derivatives (cash flow hedges) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(73)
Current - period other comprehensive income	(421)	0
Ending Balance	(494)
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(2,546)	29
Current - period other comprehensive income	6,715	3,706
Ending Balance	$ 4,169	$ 3,735