Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,587,028	$ 1,570,137	$ 1,591,453	$ 1,571,701
Other comprehensive income (loss) before reclassification			30,556	(26,114)
Amounts reclassified from accumulated other comprehensive loss			307	624
Total other comprehensive income (loss), net of tax amount	16,099	(5,853)	30,863	(25,490)
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				(8,524)
Balance	1,618,550	1,584,072	1,618,550	1,584,072
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			30,679	(26,533)
Total other comprehensive income (loss), net of tax amount			30,679	(26,533)
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				(5,694)
Balance	(13,145)	(58,762)	(13,145)	(58,762)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(12,324)	(13,468)
Amounts reclassified from accumulated other comprehensive loss			541	702
Total other comprehensive income (loss), net of tax amount			541	702
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				(2,890)
Balance	(11,783)	(15,656)	(11,783)	(15,656)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			469	278
Other comprehensive income (loss) before reclassification			(123)	419
Amounts reclassified from accumulated other comprehensive loss			(234)	(78)
Total other comprehensive income (loss), net of tax amount			(357)	341
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				60
Balance	112	679	112	679
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(40,915)	(67,886)	(55,679)	(39,725)
Balance	$ (24,816)	$ (73,739)	$ (24,816)	$ (73,739)