Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 1,520,208	$ 1,473,057
Other comprehensive income (loss) before reclassification			6,044	24,127
Amounts reclassified from accumulated other comprehensive loss			11,929	4,192
Total other comprehensive income (loss)	$ 684	$ (4,911)	17,973	28,319
Balance	1,582,535	1,534,761	1,582,535	1,534,761
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			6,105	24,967
Amounts reclassified from accumulated other comprehensive loss			(9)	191
Total other comprehensive income (loss)			6,096	25,158
Balance	(14,704)	7,764	(14,704)	7,764
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(24,980)	(27,840)
Amounts reclassified from accumulated other comprehensive loss			11,795	3,709
Total other comprehensive income (loss)			11,795	3,709
Balance	(13,185)	(24,131)	(13,185)	(24,131)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(18)	(160)
Other comprehensive income (loss) before reclassification			(61)	(840)
Amounts reclassified from accumulated other comprehensive loss			143	292
Total other comprehensive income (loss)			82	(548)
Balance	64	(708)	64	(708)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(45,798)	(45,394)
Balance	$ (27,825)	$ (17,075)	$ (27,825)	$ (17,075)