Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 1,473,057	$ 1,419,940
Other comprehensive income (loss) before reclassification			24,127	13,514
Amounts reclassified from accumulated other comprehensive loss			4,192	390
Total other comprehensive (loss) income, Net of tax	$ (4,911)	$ 12,613	28,319	13,904
Balance	1,534,761	1,476,756	1,534,761	1,476,756
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(17,394)	(11,003)
Other comprehensive income (loss) before reclassification			24,967	11,401
Amounts reclassified from accumulated other comprehensive loss			191
Total other comprehensive (loss) income, Net of tax			25,158	11,401
Balance	7,764	398	7,764	398
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(27,840)	(31,617)
Other comprehensive income (loss) before reclassification				3,298
Amounts reclassified from accumulated other comprehensive loss			3,709
Total other comprehensive (loss) income, Net of tax			3,709	3,298
Balance	(24,131)	(28,319)	(24,131)	(28,319)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(160)	136
Other comprehensive income (loss) before reclassification			(840)	(1,185)
Amounts reclassified from accumulated other comprehensive loss			292	390
Total other comprehensive (loss) income, Net of tax			(548)	(795)
Balance	(708)	(659)	(708)	(659)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(45,394)	(42,484)
Balance	$ (17,075)	$ (28,580)	$ (17,075)	$ (28,580)