Retirement Plans - Qualified Defined Benefit Pension Plans and Nonqualified Postretirement Benefit Plans (Narrative) (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) hour benefit_plan $ / shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of employee benefit plans | benefit_plan	4
Percent participant accounts are vested	100.00%
401K Plan After January 1, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Sale of stock price (USD per share) | $ / shares	$ 1
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Requisite age of employees to be eligible for participation	21 years
Requisite number of hours worked to be eligible for participation | hour	1,000
Term of treasury note used to determine rate credited to participants' accounts	5 years
Basis spread on variable base rate used to calculate interest on participants' accounts balance (as a percent)	0.25%
Monthly interest credit cap	0.12
Weighted-average interest crediting rate	3.00%	2.25%
Fair value adjustment	$ 333	$ (607)
Discount rate used to determine the projected benefit obligation	3.48%
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Requisite age of employees to be eligible for participation	25 years
Fair value adjustment	$ 0	$ 0
Legacy TCF | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value adjustment	$ 2,300
Discount rate used to determine the projected benefit obligation	2.17%	3.95%	3.30%
Legacy TCF | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate used to determine the projected benefit obligation	2.70%	3.85%	3.15%
Chemical Financial Corporation | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value adjustment	$ 4,000
Discount rate used to determine the projected benefit obligation	2.54%	0.00%	0.00%
Chemical Financial Corporation | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate used to determine the projected benefit obligation	3.14%	0.00%	0.00%
Minimum | Defined Benefit Plan, Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets, target allocation, percentage	40.00%
Minimum | Defined Benefit Plan, Debt Security
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets, target allocation, percentage	20.00%
Minimum | Cash
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets, target allocation, percentage	0.00%
Maximum | Defined Benefit Plan, Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets, target allocation, percentage	70.00%
Maximum | Defined Benefit Plan, Debt Security
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets, target allocation, percentage	60.00%
Maximum | Cash
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets, target allocation, percentage	10.00%