Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,571,701	$ 1,520,208	$ 1,473,057
Other comprehensive income (loss) before reclassification	(8,416)	(6,364)	(3,973)
Amounts reclassified from accumulated other comprehensive loss	986	12,437	3,569
Total other comprehensive income (loss), net of tax amount	(7,430)	6,073	(404)
Balance	1,591,453	1,571,701	1,520,208
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Reform Act	(8,524)
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(26,535)	(20,800)	(17,394)
Other comprehensive income (loss) before reclassification	(11,595)	(5,726)	(3,597)
Amounts reclassified from accumulated other comprehensive loss	0	(9)	191
Total other comprehensive income (loss), net of tax amount	(11,595)	(5,735)	(3,406)
Balance	(43,824)	(26,535)	(20,800)
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Reform Act	(5,694)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(13,468)	(24,980)	(27,840)
Other comprehensive income (loss) before reclassification	2,806	(760)	(148)
Amounts reclassified from accumulated other comprehensive loss	1,228	12,272	3,008
Total other comprehensive income (loss), net of tax amount	4,034	11,512	2,860
Balance	(12,324)	(13,468)	(24,980)
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Reform Act	(2,890)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	278	(18)	(160)
Other comprehensive income (loss) before reclassification	373	122	(228)
Amounts reclassified from accumulated other comprehensive loss	(242)	174	370
Total other comprehensive income (loss), net of tax amount	131	296	142
Balance	469	278	(18)
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Reform Act	60
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(39,725)	(45,798)	(45,394)
Other comprehensive income (loss) before reclassification	0	0	0
Amounts reclassified from accumulated other comprehensive loss	0	0	0
Total other comprehensive income (loss), net of tax amount	0	0	0
Balance	(55,679)	$ (39,725)	$ (45,798)
Reclassification of certain income tax effects related to the change in the federal statutory income tax rate under the Tax Reform Act	$ 0