Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,473,057	$ 1,419,940	$ 1,354,953
Other comprehensive income (loss) before reclassification	(3,973)	(3,426)	1,433
Amounts reclassified from accumulated other comprehensive loss	3,569	516	(186)
Total other comprehensive income (loss), net of tax amount	(404)	(2,910)	1,247
Balance	1,520,208	1,473,057	1,419,940
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(17,394)	(11,003)	(25,462)
Other comprehensive income (loss) before reclassification	(3,597)	(6,391)	14,644
Amounts reclassified from accumulated other comprehensive loss	191		(185)
Total other comprehensive income (loss), net of tax amount	(3,406)	(6,391)	14,459
Balance	(20,800)	(17,394)	(11,003)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(27,840)	(31,617)	(19,793)
Other comprehensive income (loss) before reclassification	(148)	3,777	(11,824)
Amounts reclassified from accumulated other comprehensive loss	3,008
Total other comprehensive income (loss), net of tax amount	2,860	3,777	(11,824)
Balance	(24,980)	(27,840)	(31,617)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(160)	136	1,524
Other comprehensive income (loss) before reclassification	(228)	(812)	(1,387)
Amounts reclassified from accumulated other comprehensive loss	370	516	(1)
Total other comprehensive income (loss), net of tax amount	142	(296)	(1,388)
Balance	(18)	(160)	136
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(45,394)	(42,484)	(43,731)
Balance	$ (45,798)	$ (45,394)	$ (42,484)