Other Comprehensive (Loss) Income - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense (benefit)		$ (9,414)	$ (9,034)	$ (9,601)	$ (8,392)	$ (9,968)	$ (7,913)	$ (6,206)	$ (7,698)	$ (36,441)	$ (31,785)	$ (35,366)
Realized loss related to lump sum pension settlement										(1,300)
Net income		$ 21,515	$ 20,609	$ 21,796	$ 19,802	$ 21,219	$ 19,032	$ 16,351	$ 17,029	83,722	73,631	70,534
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Unrealized (losses) on derivatives (cash flow hedges)										(122)	0	0
Amount Reclassified from AOCI [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income										854	(498)	1,141
Amount Reclassified from AOCI [Member] | Securities available for sale: Realized net gains [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Realized net gains on the sale of securities available for sale										654	251	996
Income tax expense (benefit)										(263)	(101)	(407)
Net income										391	150	589
Amount Reclassified from AOCI [Member] | Securities available for sale: Realized other-than-temporary impairment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense (benefit)										0	0	177
Realized other-than-temporary impairment losses on securities available for sale										0	0	(434)
Net income										0	0	(257)
Amount Reclassified from AOCI [Member] | Post-retirement obligations [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense (benefit)										(311)	(95)	(558)
Amortization of actuarial losses (gains)	[1]									774	237	1,367
Net income										463	142	809
Amount Reclassified from AOCI [Member] | Realized loss related to lump sum pension settlement [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense (benefit)										0	546	0
Realized loss related to lump sum pension settlement	[1]									0	(1,336)	0
Net income										$ 0	$ (790)	$ 0

[1] This item is included in the computation of net periodic benefit cost. See Note 11. Benefit Plans