Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Debt Outstanding [Line Items]
Short-term borrowings	$ 3,138,162	$ 17,430
Long-term debt	$ 348,076	347,987
Senior Notes | 3.50% Senior Notes
Debt Outstanding [Line Items]
Maturity Date	Jan. 29, 2025
Principal value at period end	$ 350,000
Long-term debt	$ 348,076	$ 347,987
Interest rate on senior notes	3.50%	3.50%
Short-term FHLB advances
Debt Outstanding [Line Items]
Principal value at period end	$ 2,700,000
Short-term borrowings	2,700,000	$ 0
Federal funds purchased
Debt Outstanding [Line Items]
Principal value at period end	375,000
Short-term borrowings	375,000	0
Other short-term borrowings
Debt Outstanding [Line Items]
Principal value at period end	63,162
Short-term borrowings	$ 63,162	$ 17,430