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Retirement Plans - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Defined Benefit Pension Plans          
Defined Benefit Plan Disclosure [Line Items]          
Weighted-average interest crediting rate 2.05%   2.05%    
Discount rate 3.48%   3.48%    
Company contributions to defined benefit plans $ 0 $ 0 $ 0   $ 0
Postretirement Benefit Plans          
Defined Benefit Plan Disclosure [Line Items]          
Company contributions to defined benefit plans 100,000 100,000 $ 100,000 $ 200,000  
Chemical Financial Corporation          
Defined Benefit Plan Disclosure [Line Items]          
Company contributions to defined benefit plans 0        
401K Plan After January 1, 2016          
Defined Benefit Plan Disclosure [Line Items]          
Maximum percent of eligible compensation for 401k plan     50.00%    
Employer matching contribution, per dollar     1    
The TCF 401K Plan          
Defined Benefit Plan Disclosure [Line Items]          
Company contributions to 401k Plans $ 5,300,000 $ 2,700,000 $ 11,800,000 $ 7,100,000  
Maximum | 401K Plan After January 1, 2016          
Defined Benefit Plan Disclosure [Line Items]          
Employer matching contribution, percent     5.00%