Retirement Plans (Textual) (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) employee shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Common Stock, Shares Held by Plan | shares		71,207,114	70,599,133
Dividends paid (in dollars per share)		$ 78,498	$ 49,389	$ 36,918
Multiemployer Plan, Contributions by Employer		0
Multiemployer Plan, Participating Company, Allocation of Plan Liabilities		$ 3,700	2,600
Multiemployer Plan, Company's Asset Participation Level		0.10%
Employer discretionary contribution percent		4.00%
Employer total contribution amount		$ 9,100	6,500	4,700
Contribution to 401(k) savings plan		$ 6,200	3,500	3,100
Maximum annual contribution per employee percent		50.00%
Employer matching contribution percent		4.00%
Employer matching contribution amount		$ 2,900	3,000	1,600
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Number of Employees Covered (in employee) | employee		100
Pension Plan expense		$ (1,495)	73	$ 1,575
Employer contributions		$ 0	$ 0
Discount rate used in determining benefit obligation		3.68%	4.22%	4.55%
Discount rate used in determining expense		4.22%	4.55%	4.15%
Curtailment	$ 100	$ 11,911	$ 0
Pension plan actual asset allocations		100.00%	100.00%
Projected benefit obligation		$ 121,361	$ 123,968	$ 119,873
Accumulated benefit obligation		121,361	115,418
Settlement		$ 0	$ 0	0
Pension Plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan actual asset allocations		68.00%	69.00%
Pension Plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan actual asset allocations		28.00%	29.00%
Pension Plan | Other
Defined Benefit Plan Disclosure [Line Items]
Pension plan actual asset allocations		4.00%	2.00%
Pension Plan | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Common Stock, Shares Held by Plan | shares		0	210,663.000
Fair value of common stock		$ 0	$ 11,400
Pension plan actual asset allocations		0.00%	8.60%
SERP
Defined Benefit Plan Disclosure [Line Items]
Pension Plan expense		$ 529	$ 367	$ 417
Employer contributions		$ 0	$ 0
Discount rate used in determining benefit obligation		3.38%	3.63%	4.51%
Discount rate used in determining expense		0.00%	4.51%	4.07%
Curtailment		$ 0	$ 0
Number of employees covered by the pension plan | employee		1
Projected benefit obligation		$ 2,423	2,567	$ 2,160
Accumulated benefit obligation		2,423	2,567
Settlement		(322)	(120)	0
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Pension Plan expense		(63)	158	$ 278
Employer contributions		$ 198	$ 309
Discount rate used in determining benefit obligation		3.41%	3.79%	4.23%
Discount rate used in determining expense		3.79%	4.23%	3.75%
Curtailment		$ 0	$ 0
Expected health care cost trend rate assumed for 2014		7.50%
Expected ultimate health care cost trend rate in 2020		5.00%
Projected benefit obligation		$ 2,512	2,601	$ 3,247
Accumulated benefit obligation		2,512	2,601
Settlement		$ 0	0	$ 0
Minimum retirement age (years)		50 years
Maximum benefit age of spouse (years)		65 years
Minimum retirement age with twenty-five years of service to be eligible for postretirement benefits (years)		60 years
Minimum years of service for retirees at age sixty to be eligible for postretirement benefits (years)		25 years
Chemical Financial Corporation | Pension Plan | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Dividends paid (in dollars per share)		$ 200	$ 200
Minimum | Pension Plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range		40.00%
Minimum | Pension Plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range		20.00%
Minimum | Pension Plan | Other
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range		0.00%
Maximum | Pension Plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range		70.00%
Maximum | Pension Plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range		60.00%
Maximum | Pension Plan | Other
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range		10.00%
Qualified Plan | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate used in determining expense	3.68%
Nonqualified Plan | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefits owed related to change in control		$ 2,400