Accumulated Other Comprehensive Loss - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sale of investment securities	$ 4,160	$ 3	$ 4,247	$ 3
Income tax expense	(15,226)	(12,434)	(28,891)	(25,389)
Net income	69,594	68,988	132,536	140,584
Interest on short-term borrowings (interest expense)	165,161	157,537	327,985	309,400
Gains and losses on investment securities carried at fair value | Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sale of investment securities	4,160	3	4,247	3
Income tax expense	(874)	(1)	(892)	(1)
Net income	3,286	2	3,355	2
Amortization of defined benefit pension plan items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, wages and employee benefits (operating expenses)	96	142	191	284
Income tax expense	(20)	(30)	(40)	(60)
Net Loss	76	112	151	224
Gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	264	124	534	73
Gains and losses on cash flow hedges | Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(992)	(465)	(2,010)	(274)
Interest on short-term borrowings (interest expense)	$ (1,256)	$ (589)	$ (2,544)	$ (347)