Merger - Narrative (Details) - USD ($)		3 Months Ended
	Aug. 01, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Common stock, shares outstanding (in shares)	153,500,000
Interest income		$ 495,392,000	$ 306,894,000
Other intangible assets	$ 159,532,000
Estimated useful life		11 years
Goodwill	1,158,289,000	$ 1,313,046,000		$ 1,299,878,000
Financing Receivable, Allowance for Credit Loss		$ 406,383,000	$ 147,972,000	113,052,000	$ 157,446,000
Core deposit intangibles
Business Acquisition [Line Items]
Other intangible assets	138,200,000
Estimated useful life		10 years
Customer relationships
Business Acquisition [Line Items]
Other intangible assets	$ 21,300,000
Estimated useful life		15 years 7 months 6 days
Legacy TCF
Business Acquisition [Line Items]
Common stock, shares outstanding (in shares)	302,065,045
Chemical Financial Corporation
Business Acquisition [Line Items]
Common stock, shares outstanding (in shares)	153,479,249
Financing Receivable, Allowance for Credit Loss	$ 0
TCF Financial Corporation
Business Acquisition [Line Items]
Voting interest percent	53.00%
Chemical Common Stock
Business Acquisition [Line Items]
Share conversion ratio	0.5081
Shares issued in acquisition (in shares)	81,900,000
Series C Preferred Stock
Business Acquisition [Line Items]
Annual rate of dividend (as a percent)		5.70%	5.70%
Series C Preferred Stock | Legacy TCF
Business Acquisition [Line Items]
Annual rate of dividend (as a percent)	5.70%
Liquidation preference (in usd per share)	$ 25,000
New Chemical Preferred Stock | Chemical Financial Corporation
Business Acquisition [Line Items]
Share conversion ratio	1
Annual rate of dividend (as a percent)	5.70%
Liquidation preference (in usd per share)	$ 25,000
Preferred stock, shares outstanding (in shares)	7,000
Depositary Shares | Chemical Financial Corporation
Business Acquisition [Line Items]
Preferred stock, shares outstanding (in shares)	7,000,000.0
Consumer Banking
Business Acquisition [Line Items]
Goodwill	$ 528,000,000.0	$ 771,555,000		$ 764,389,000
Commercial Banking
Business Acquisition [Line Items]
Goodwill	630,300,000
Pro Forma
Business Acquisition [Line Items]
Interest income	$ 2,400,000