Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Fixed interest rate commitments	$ 2,474,804		$ 2,434,042
Variable interest rate commitments	24,796,923		19,309,317
Total loan commitments available for funding	27,271,727		21,743,359
Commercial and standby letters of credit	3,058,069		2,778,561
Total unfunded credit commitments	30,329,796		24,521,920
Commitments unavailable for funding	2,519,517		3,051,075
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses: loans	512,958	$ 589,828	304,924	$ 304,410	$ 301,888	$ 280,903
Unfunded Credit Commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses: loans	$ 101,515	$ 99,294	$ 67,656	$ 63,108	$ 62,664	$ 55,183