Accumulated Other Comprehensive Loss - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sale of investment securities	$ 3	$ 77	$ 3	$ 167
Income tax expense	(12,434)	(23,036)	(25,389)	(35,293)
Net income	68,988	52,014	140,584	99,618
Interest on short-term borrowings (interest expense)	157,537	137,948	309,400	268,045
Gains on available-for-sale securities | 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	3	77	3	167
Income tax expense	(1)	(27)	(1)	(58)
Net income	2	50	2	109
Amortization of defined benefit pension plan items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, wages and employee benefits (operating expenses)	142	539	284	1,076
Income tax benefit	(30)	(189)	(60)	(377)
Net Loss	112	350	224	699
Gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit	(124)	57	(73)	57
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	465	(409)	274	(409)
Interest on short-term borrowings (interest expense)	$ 589	$ (466)	$ 347	$ (466)