Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,520,208	$ 1,473,057	$ 1,419,940
Other comprehensive income (loss) before reclassification	(6,364)	(3,973)	(3,426)
Amounts reclassified from accumulated other comprehensive loss	12,437	3,569	516
Total other comprehensive income (loss), net of tax amount	6,073	(404)	(2,910)
Balance	1,571,701	1,520,208	1,473,057
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(20,800)	(17,394)	(11,003)
Other comprehensive income (loss) before reclassification	(5,726)	(3,597)	(6,391)
Amounts reclassified from accumulated other comprehensive loss	(9)	191
Total other comprehensive income (loss), net of tax amount	(5,735)	(3,406)	(6,391)
Balance	(26,535)	(20,800)	(17,394)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(24,980)	(27,840)	(31,617)
Other comprehensive income (loss) before reclassification	(760)	(148)	3,777
Amounts reclassified from accumulated other comprehensive loss	12,272	3,008
Total other comprehensive income (loss), net of tax amount	11,512	2,860	3,777
Balance	(13,468)	(24,980)	(27,840)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(18)	(160)	136
Other comprehensive income (loss) before reclassification	122	(228)	(812)
Amounts reclassified from accumulated other comprehensive loss	174	370	516
Total other comprehensive income (loss), net of tax amount	296	142	(296)
Balance	278	(18)	(160)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(45,798)	(45,394)	(42,484)
Balance	$ (39,725)	$ (45,798)	$ (45,394)