Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,584,072	$ 1,561,918	$ 1,584,072	$ 1,561,918	$ 1,571,701	$ 1,520,208
Other comprehensive income (loss) before reclassification			(26,114)	5,644
Amounts reclassified from accumulated other comprehensive loss			624	11,645
Total other comprehensive income (loss), net of tax	(5,853)	14,496	(25,490)	17,289
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	(58,762)	(15,063)	(58,762)	(15,063)	(26,535)	(20,800)
Other comprehensive income (loss) before reclassification			(26,533)	5,738
Amounts reclassified from accumulated other comprehensive loss				(1)
Total other comprehensive income (loss), net of tax			(26,533)	5,737
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	(15,656)	(13,443)	(15,656)	(13,443)	(13,468)	(24,980)
Amounts reclassified from accumulated other comprehensive loss			702	11,537
Total other comprehensive income (loss), net of tax			702	11,537
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	679	(3)	679	(3)	278	(18)
Other comprehensive income (loss) before reclassification			419	(94)
Amounts reclassified from accumulated other comprehensive loss			(78)	109
Total other comprehensive income (loss), net of tax			341	15
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	$ (73,739)	$ (28,509)	$ (73,739)	$ (28,509)	$ (39,725)	$ (45,798)