Accumulated Other Comprehensive Loss - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sale of investment securities	$ 0	$ 1	$ 3	$ 168
Income tax expense	(11,312)	(10,253)	(36,701)	(45,546)
Net income	70,397	40,459	210,981	140,077
Interest on short-term borrowings (interest expense)	159,481	143,628	468,881	411,673
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	0	1	3	168
Income tax expense	0	0	(1)	(59)
Net income	0	1	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	537	426	1,613
Income tax benefit	(30)	(188)	(90)	(565)
Net Loss	112	349	336	1,048
Gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit	139	(344)	212	(344)
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	(523)	231	(797)	640
Interest on short-term borrowings (interest expense)	$ (662)	$ 575	$ (1,009)	$ 984