Business Combinations - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Dec. 13, 2020	Aug. 01, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Total assets			$ 47,802,487	$ 46,651,553	$ 23,699,612
Common stock, shares outstanding (in shares)		153,500,000
Other intangible assets		$ 159,532
Estimated useful life			11 years
Goodwill		1,158,289	$ 1,313,046	1,299,878
Consumer Banking
Business Acquisition [Line Items]
Total assets			13,663,990	14,224,545	6,414,228
Goodwill		528,000	771,555	764,389
Commercial Banking
Business Acquisition [Line Items]
Total assets			$ 24,063,599	20,395,308	$ 9,086,125
Goodwill		630,300
Core deposit intangibles
Business Acquisition [Line Items]
Other intangible assets		138,200
Estimated useful life			10 years
Customer relationships
Business Acquisition [Line Items]
Other intangible assets		$ 21,300
Estimated useful life			15 years 7 months 6 days
Pro Forma
Business Acquisition [Line Items]
Interest income				$ 2,400
Common Stock
Business Acquisition [Line Items]
Share conversion ratio (in shares)	3.0028	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%
Liquidation preference (in usd per share)			$ 25,000
Huntington Bancshares Incorporated
Business Acquisition [Line Items]
Total assets			$ 123,000,000
Huntington Bancshares Incorporated | New Chemical Preferred Stock
Business Acquisition [Line Items]
Share conversion ratio (in shares)	1
Annual rate of dividend (as a percent)	5.70%
Legacy TCF
Business Acquisition [Line Items]
Common stock, shares outstanding (in shares)		302,065,045
Legacy TCF | Series C Preferred Stock
Business Acquisition [Line Items]
Annual rate of dividend (as a percent)		5.70%
Liquidation preference (in usd per share)		$ 25,000
TCF Financial Corporation
Business Acquisition [Line Items]
Voting interest percent		53.00%
TCF Financial Corporation | New Chemical Preferred Stock
Business Acquisition [Line Items]
Share conversion ratio (in shares)		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
TCF Financial Corporation | Depositary Shares
Business Acquisition [Line Items]
Preferred stock, shares outstanding (in shares)		7,000,000.0