Other Comprehensive (Loss) Income - Schedule of Components (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Unrealized gains and losses on securities available for sale:
Net (losses) gains arising during the period	$ (5,683)	$ 17,868	$ (32,845)
Reclassification adjustment for gains included in net income	(654)	(251)	(996)
Total	(6,337)	17,617	(33,841)
Other-than-temporary impairment on debt securities available for sale:
Other-than-temporary impairment losses on securities	0	0	0
Reclassification adjustment for impairment losses included in net income	0	0	434
Total	0	0	434
Amortization related to post retirement obligations	2,156	(9,462)	12,161
Total other comprehensive (loss) income	(4,303)	8,155	(21,246)
Unrealized gains and losses on securities available for sale:
Net (losses) gains arising during the period	2,282	(7,176)	13,417
Reclassification adjustment for gains included in net income	263	101	407
Total	2,545	(7,075)	13,824
Other-than-temporary impairment on debt securities available for sale:
Other-than-temporary impairment losses on securities	0	0	0
Reclassification adjustment for impairment losses included in net income	0	0	(177)
Total	0	0	(177)
Amortization related to post retirement obligations	(866)	3,800	(4,968)
Total other comprehensive (loss) income	1,728	(3,275)	8,679
Unrealized gains and losses on securities available for sale:
Net (losses) gains arising during the period	(3,401)	10,692	(19,428)
Reclassification adjustment for gains included in net income	(391)	(150)	(589)
Total	(3,792)	10,542	(20,017)
Other-than-temporary impairment on debt securities available for sale:
Other-than-temporary impairment losses on securities	0	0	0
Reclassification adjustment for impairment losses included in net income	0	0	257
Total	0	0	257
Amortization related to post retirement obligations	1,290	(5,662)	7,193
Total other comprehensive (loss) income	(2,575)	4,880	(12,567)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Other-than-temporary impairment on debt securities available for sale:
Unrealized (losses) on derivatives (cash flow hedges)	(122)	0	0
Other-than-temporary impairment on debt securities available for sale:
Unrealized (losses) on derivatives (cash flow hedges)	49	0	0
Other-than-temporary impairment on debt securities available for sale:
Unrealized (losses) on derivatives (cash flow hedges)	(73)	0	$ 0
Total other comprehensive (loss) income	$ (3,792)	$ 10,542