Merger - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 01, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Common stock, shares outstanding (in shares)	153,500,000
Carryover of the related allowance for loans and lease losses		$ 515,229,000	$ 121,218,000	$ 515,229,000	$ 121,218,000	$ 113,052,000	$ 461,114,000	$ 146,503,000	$ 157,446,000
Interest income		420,448,000	$ 459,808,000	$ 1,354,210,000	$ 1,077,483,000
Other intangible assets	$ 159,532,000
Estimated useful life				11 years
Goodwill	1,158,289,000	$ 1,313,046,000		$ 1,313,046,000		1,299,878,000
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
Carryover of the related allowance for loans and lease losses	$ 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%	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		$ 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