Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 1,520,208	$ 1,473,057
Other comprehensive income (loss) before reclassification			5,644	30,033
Amounts reclassified from accumulated other comprehensive loss			11,645	3,197
Total other comprehensive income (loss)	$ 14,496	$ 9,409	17,289	33,230
Balance	1,561,918	1,523,467	1,561,918	1,523,467
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			5,738	31,130
Amounts reclassified from accumulated other comprehensive loss			(1)	191
Total other comprehensive income (loss)			5,737	31,321
Balance	(15,063)	13,927	(15,063)	13,927
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(24,980)	(27,840)
Amounts reclassified from accumulated other comprehensive loss			11,537	2,811
Total other comprehensive income (loss)			11,537	2,811
Balance	(13,443)	(25,029)	(13,443)	(25,029)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(18)	(160)
Other comprehensive income (loss) before reclassification			(94)	(1,097)
Amounts reclassified from accumulated other comprehensive loss			109	195
Total other comprehensive income (loss)			15	(902)
Balance	(3)	(1,062)	(3)	(1,062)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(45,798)	(45,394)
Balance	$ (28,509)	$ (12,164)	$ (28,509)	$ (12,164)