Retirement Plans - Nonqualified Supplemental Retirement Plans (Details) - TCF 401K Supplemental Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure
Maximum employee contribution as a percent of covered compensation	50.00%
Contributions to the plan	$ 0.5	$ 1.2	$ 1.3
Fair value of assets in the plan	69.5	69.7
Common Stock
Defined Contribution Plan Disclosure
Fair value of assets in the plan	$ 21.5	$ 27.6