Merger - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Aug. 01, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Business Acquisition [Line Items]
Total consideration	$ 21,743,574
Common stock, shares outstanding (in shares)	153,500,000	153,571,381		153,571,381		88,198,460
Preferred stock, shares outstanding (in shares)		7,000		7,000		7,000
Other intangible assets	$ 201,568
Estimated useful life				11 years
Goodwill	1,110,354	$ 1,265,111		$ 1,265,111		$ 154,757
Core deposit intangibles
Business Acquisition [Line Items]
Other intangible assets	177,800
Estimated useful life				10 years
Customer relationships
Business Acquisition [Line Items]
Other intangible assets	$ 23,800
Estimated useful life				15 years 7 months 6 days
Legacy TCF
Business Acquisition [Line Items]
Common stock, shares outstanding (in shares)	302,065,000
Chemical Financial Corporation
Business Acquisition [Line Items]
Common stock, shares outstanding (in shares)	153,479,000
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