Other Comprehensive Income (Components of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Unrealized gains and losses on securities available for sale:
Net gains arising during the period	$ 1,287	$ 11,856
Reclassification adjustment for gains included in net income	0	(96)
Total	1,287	11,760
Unrealized gains (losses) on derivatives (cash flow hedges)	92	(704)
Amortization related to post-retirement obligations	53	165
Total other comprehensive income	1,432	11,221
Unrealized gains and losses on securities available for sale:
Net gains arising during the period	(516)	(4,762)
Reclassification adjustment for gains included in net income	0	39
Total	(516)	(4,723)
Unrealized gains (losses) on derivatives (cash flow hedges)	(37)	283
Amortization related to post-retirement obligations	(21)	(66)
Total other comprehensive income	(574)	(4,506)
Unrealized gains and losses on securities available for sale:
Net gains arising during the period	771	7,094
Reclassification adjustment for gains included in net income	0	(57)
Total	771	7,037
Unrealized gains (losses) on derivatives (cash flow hedges)	55	(421)
Amortization related to post-retirement obligations	32	99
Total other comprehensive income	$ 858	$ 6,715