Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,618,550	$ 1,584,072	$ 1,591,453	$ 1,571,701
Other comprehensive income (loss) before reclassification			34,323	(32,591)
Amounts reclassified from accumulated other comprehensive loss			498	894
Other comprehensive income (loss), net of tax amount	3,958	(6,207)	34,821	(31,697)
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				(8,524)
Balance	1,645,362	1,599,604	1,645,362	1,599,604
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			34,432	(33,049)
Other comprehensive income (loss), net of tax amount			34,432	(33,049)
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				(5,694)
Balance	(9,392)	(65,278)	(9,392)	(65,278)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(12,324)	(13,468)
Amounts reclassified from accumulated other comprehensive loss			807	1,051
Other comprehensive income (loss), net of tax amount			807	1,051
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				(2,890)
Balance	(11,517)	(15,307)	(11,517)	(15,307)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			469	278
Other comprehensive income (loss) before reclassification			(109)	458
Amounts reclassified from accumulated other comprehensive loss			(309)	(157)
Other comprehensive income (loss), net of tax amount			(418)	301
Reclassification of certain income tax effects related to the changein the federal statutory income tax rate under the Tax Reform Act				60
Balance	51	639	51	639
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(24,816)	(73,739)	(55,679)	(39,725)
Balance	$ (20,858)	$ (79,946)	$ (20,858)	$ (79,946)