Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,599,604	$ 1,582,535	$ 1,599,604	$ 1,582,535	$ 1,571,701	$ 1,520,208
Other comprehensive income (loss) before reclassification			(32,591)	6,044
Amounts reclassified from accumulated other comprehensive loss			894	11,929
Total other comprehensive income (loss), net of tax amount	(6,207)	684	(31,697)	17,973
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Cuts and Jobs Act of 2017 (Tax Reform Act)			(8,524)
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(65,278)	(14,704)	(65,278)	(14,704)	(26,535)	(20,800)
Other comprehensive income (loss) before reclassification			(33,049)	6,105
Amounts reclassified from accumulated other comprehensive loss				(9)
Total other comprehensive income (loss), net of tax amount			(33,049)	6,096
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Cuts and Jobs Act of 2017 (Tax Reform Act)			(5,694)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(15,307)	(13,185)	(15,307)	(13,185)	(13,468)	(24,980)
Amounts reclassified from accumulated other comprehensive loss			1,051	11,795
Total other comprehensive income (loss), net of tax amount			1,051	11,795
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Cuts and Jobs Act of 2017 (Tax Reform Act)			(2,890)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	639	64	639	64	278	(18)
Other comprehensive income (loss) before reclassification			458	(61)
Amounts reclassified from accumulated other comprehensive loss			(157)	143
Total other comprehensive income (loss), net of tax amount			301	82
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Cuts and Jobs Act of 2017 (Tax Reform Act)			60
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (79,946)	$ (27,825)	$ (79,946)	$ (27,825)	$ (39,725)	$ (45,798)