Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,591,453	$ 1,571,701
Other comprehensive income (loss) before reclassification	14,631	(19,986)
Amounts reclassified from accumulated other comprehensive loss	133	349
Total other comprehensive income (loss), net of tax amount	14,764	(19,637)
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act		(8,524)
Balance	1,587,028	1,570,137
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(43,824)	(26,535)
Other comprehensive income (loss) before reclassification	14,678	(20,306)
Total other comprehensive income (loss), net of tax amount	14,678	(20,306)
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act		(5,694)
Balance	(29,146)	(52,535)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(12,324)	(13,468)
Amounts reclassified from accumulated other comprehensive loss	261	354
Total other comprehensive income (loss), net of tax amount	261	354
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act		(2,890)
Balance	(12,063)	(16,004)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	469	278
Other comprehensive income (loss) before reclassification	(47)	320
Amounts reclassified from accumulated other comprehensive loss	(128)	(5)
Total other comprehensive income (loss), net of tax amount	(175)	315
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act		60
Balance	294	653
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(55,679)	(39,725)
Balance	$ (40,915)	$ (67,886)