Accumulated Other Comprehensive Income (Loss) - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Other income - net realized gains on sales of available for sale securities	$ 332	$ 839	$ 1,497	$ 2,542
Interest expense - subordinated debentures and term loans	1,945	1,797	5,602	5,368
Amortization of prior service costs	33,244	26,651	86,052	79,558
Income tax expense	(7,939)	(7,469)	(22,314)	(19,759)
Total reclassifications for the period, net of tax	118	339	655	1,029
Unrealized gains (losses) on available for sale securities
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	216	545	973	1,652
Unrealized gains (losses) on cash flow hedges
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	(156)	(206)	(493)	(623)
Unrealized Gains (Losses) on Defined Benefit Plans
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax			175
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized gains (losses) on available for sale securities
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Other income - net realized gains on sales of available for sale securities	332	839	1,497	2,542
Income tax expense	(116)	(294)	(524)	(890)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized gains (losses) on cash flow hedges
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	84	111	266	336
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized gains (losses) on cash flow hedges | Interest rate contracts
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Interest expense - subordinated debentures and term loans	(240)	(317)	(759)	(959)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized Gains (Losses) on Defined Benefit Plans
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs	89		269
Income tax expense	(31)		(94)
Total reclassifications for the period, net of tax	$ 58	$ 0	$ 175	$ 0