Summary of Significant Accounting Policies - Narrative (Details) - USD ($)		9 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Change in Accounting Principle
Debt Securities, Held-to-maturity, Allowance for Credit Loss		$ 0
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Allowance for credit loss		$ 515,229,000	$ 461,114,000	$ 113,052,000	$ 121,218,000	$ 146,503,000	$ 157,446,000
Projected increase in allowance for loan losses, percent	20.00%
Allowance for loan and lease losses	80.00%
Reclassification into nonaccrual loans	$ 73,400,000	376,721,000		169,658,000
Reduction in retained earnings		$ 1,700,044,000		1,896,427,000
Forecast period		24 months
Historical loss reversion period		12 months
Debt Securities, Available-for-sale, Allowance for Credit Loss		$ 0
Cumulative Effect, Period of Adoption, Adjustment
Change in Accounting Principle
Allowance for credit loss	206,000,000.0			205,992,000
Reclassification of previous purchased credit impaired loans	(73,400,000)
Unfunded Loan Commitment | Cumulative Effect, Period of Adoption, Adjustment
Change in Accounting Principle
Allowance for credit loss	14,700,000
TCF
Change in Accounting Principle
Total capital	$ (159,300,000)	$ 4,972,715,000		$ 4,681,630,000