Commitments, Contingent Liabilities and Guarantees - Off-Balance Sheet Risks (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	$ 6,676,052	$ 6,637,154
Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	6,555,273	6,522,518
Standby letters of credit and guarantees on industrial revenue bonds
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	120,779	114,636
Commercial | Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	4,441,962	4,396,191
Consumer | Total commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet risks	$ 2,113,311	$ 2,126,327