Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ (54,679)
Personnel expense	419,778	$ 414,837	$ 404,741
Interest income (amortization of net unrealized losses (gains) on available for sale securities transferred to held to maturity securities)	(2,665)	(5,887)	(555)
Total other comprehensive income (loss)	(8,079)	(22,063)	(27,766)
Ending Balance	(62,758)	(54,679)
Investments Securities Available For Sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(20,079)	459	18,512
Other comprehensive income (loss) before reclassifications	(27,040)	(17,900)	(20,439)
Investment securities gain (loss), net		(9,316)	(8,133)
Personnel expense	0	0	0
Interest income (amortization of net unrealized losses (gains) on available for sale securities transferred to held to maturity securities)	(2,665)	(5,887)	(555)
Income tax (expense) benefit	11,331	12,565	11,074
Total other comprehensive income (loss)	(18,374)	(20,538)	(18,053)
Ending Balance	(38,453)	(20,079)	459
Defined Benefit Pension and Postretirement Obligations
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(34,600)	(33,075)	(23,362)
Other comprehensive income (loss) before reclassifications	14,273	(6,141)	(17,892)
Investment securities gain (loss), net		0	0
Personnel expense	2,134	3,801	2,306
Interest income (amortization of net unrealized losses (gains) on available for sale securities transferred to held to maturity securities)	0	0	0
Income tax (expense) benefit	(6,112)	815	5,873
Total other comprehensive income (loss)	10,295	(1,525)	(9,713)
Ending Balance	(24,305)	(34,600)	(33,075)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(54,679)	(32,616)	(4,850)
Other comprehensive income (loss) before reclassifications	(12,767)	(24,041)	(38,331)
Investment securities gain (loss), net		(9,316)	(8,133)
Personnel expense	2,134	3,801	2,306
Interest income (amortization of net unrealized losses (gains) on available for sale securities transferred to held to maturity securities)	(2,665)	(5,887)	(555)
Income tax (expense) benefit	5,219	13,380	16,947
Total other comprehensive income (loss)	(8,079)	(22,063)	(27,766)
Ending Balance	$ (62,758)	$ (54,679)	$ (32,616)