Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Loan commitments available for funding	$ 30,923,217	$ 28,975,133
Commercial and standby letters of credit	3,063,410	3,007,118
Total unfunded credit commitments	33,986,627	31,982,251
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses: loans	391,751	447,765	$ 548,963	$ 304,924
Unfunded Credit Commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses: loans	$ 104,750	$ 120,796	$ 84,690	$ 67,656