Retirement Plans - Weighted Average Rate Assumptions (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 31, 2019
Pension Plan
Assumptions used to determine benefit obligation
Discount rate used in determining benefit obligation -December 31				3.48%
Pension Plan \| Legacy TCF
Assumptions used to determine benefit obligation
Discount rate used in determining benefit obligation -December 31		2.17%	3.95%
Discount rate used in determining expense	2.17%	3.95%	3.30%
Expected long-term return on Pension Plan Assets	0.50%	1.75%	1.50%
Health care cost trend rate assumed for next year	0.00%	0.00%	0.00%
Final health care cost trend rate	0.00%	0.00%	0.00%
Pension Plan \| Chemical Financial Corporation
Assumptions used to determine benefit obligation
Discount rate used in determining benefit obligation -December 31		2.54%	0.00%
Discount rate used in determining expense	2.54%	3.48%	0.00%
Expected long-term return on Pension Plan Assets	2.54%	3.48%	0.00%
Health care cost trend rate assumed for next year	0.00%	0.00%	0.00%
Final health care cost trend rate	0.00%	0.00%	0.00%
Postretirement Plan \| Legacy TCF
Assumptions used to determine benefit obligation
Discount rate used in determining benefit obligation -December 31	1.57%	2.70%	3.85%
Discount rate used in determining expense	2.70%	3.85%	3.15%
Health care cost trend rate assumed for next year	5.34%	5.45%	5.60%
Final health care cost trend rate	4.50%	4.50%	4.50%
Postretirement Plan \| Chemical Financial Corporation
Assumptions used to determine benefit obligation
Discount rate used in determining benefit obligation -December 31	2.14%	3.14%	0.00%
Discount rate used in determining expense	3.14%	3.11%	0.00%
Health care cost trend rate assumed for next year	6.75%	0.00%	0.00%
Final health care cost trend rate	4.75%	0.00%	0.00%

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

+ References

+ Details

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- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

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- References

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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

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+ Details

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