Commitments, Contingent Liabilities and Guarantees - Off-Balance Sheet Risks (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	$ 7,144,680	$ 8,177,360
Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	7,028,190	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	116,490	129,192
Commercial | Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	4,784,205	5,743,072
Consumer | Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	$ 2,243,985	$ 2,305,096