Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands		6 Months Ended
	Jan. 01, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Change in Accounting Principle
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Allowance for credit loss		$ 461,114	$ 406,383	$ 113,052	$ 146,503	$ 147,972	$ 157,446
Projected increase in allowance for loan losses, percent	20.00%
Allowance for loan and lease losses	80.00%
Reclassification into nonaccrual loans	$ 73,400	291,451		169,658
Reduction in retained earnings		$ 1,700,480		1,896,427
Forecast period		24 months
Historical loss reversion period		12 months
Cumulative Effect, Period of Adoption, Adjustment
Change in Accounting Principle
Allowance for credit loss	206,000			205,992
Reclassification of previous purchased credit impaired loans	(73,400)
Unfunded Loan Commitment | Cumulative Effect, Period of Adoption, Adjustment
Change in Accounting Principle
Allowance for credit loss	14,700
TCF
Change in Accounting Principle
Total capital	$ (159,300)	$ 4,907,760		$ 4,681,630