Accumulated Other Comprehensive Income (Loss) - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains on sales of available for sale securities	$ 1,285	$ 332	$ 4,016	$ 1,497
Interest expense - subordinated debentures and term loans	2,089	1,945	6,136	5,602
Amortization of prior service costs	32,936	33,244	97,354	86,052
Income tax expense	(8,478)	(7,939)	(23,050)	(22,314)
Total reclassifications for the period, net of tax	942	118	2,882	655
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	1,015	216	3,173	973
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	(73)	(156)	(291)	(493)
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]
Net realized gains on sales of available for sale securities	1,285	332	4,016	1,497
Income tax expense	(270)	(116)	(843)	(524)
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	20	84	77	266
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	(93)	(240)	(368)	(759)
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	$ 0	$ 58	$ 0	$ 175