Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,520,208	$ 1,473,057
Other comprehensive income (loss) before reclassification	2,207	22,687
Amounts reclassified from accumulated other comprehensive loss	586	1,134
Total other comprehensive income (loss), net of tax amount	2,793	23,821
Balance	1,537,961	1,508,256
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(20,800)	(17,394)
Other comprehensive income (loss) before reclassification	2,172	23,507
Amounts reclassified from accumulated other comprehensive loss		191
Total other comprehensive income (loss), net of tax amount	2,172	23,698
Balance	(18,628)	6,304
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(24,980)	(27,840)
Amounts reclassified from accumulated other comprehensive loss	525	844
Total other comprehensive income (loss), net of tax amount	525	844
Balance	(24,455)	(26,996)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(18)	(160)
Other comprehensive income (loss) before reclassification	35	(820)
Amounts reclassified from accumulated other comprehensive loss	61	99
Total other comprehensive income (loss), net of tax amount	96	(721)
Balance	78	(881)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(45,798)	(45,394)
Balance	$ (43,005)	$ (21,573)