Commitments, Contingent Liabilities and Guarantees - Off-Balance Sheet Risks (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	$ 6,810,596	$ 8,177,360
Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	6,693,804	8,048,168
Standby letters of credit and guarantees on industrial revenue bonds
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	116,792	129,192
Commercial | Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	4,522,128	5,743,072
Consumer | Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	$ 2,171,676	$ 2,305,096