Regulatory Capital and Reserve Requirements (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended											12 Months Ended
	Jan. 23, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum average regulatory cash reserve balance		$ 71,700											$ 71,700	$ 52,300
Leverage Ratio:
Cash dividends declared per share (in dollars per share)		$ 0.27	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 0.26	$ 0.26	$ 0.26	$ 0.26	$ 0.24	$ 0.24	$ 1.10	$ 1.06	$ 1.0
Subsequent Event
Leverage Ratio:
Cash dividends declared per share (in dollars per share)	$ 0.28
Chemical Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Risk weighted assets		$ 14,740,000											$ 14,740,000	$ 13,420,000
Total Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,614,046											$ 1,614,046	$ 1,543,018
Actual, Ratio		11.00%											11.00%	11.50%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,179,076											$ 1,179,076	$ 1,073,431
Minimum Required for Capital Adequacy Purposes, Ratio		8.00%											8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,363,307											$ 1,363,307	$ 1,157,293
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.30%											9.30%	8.60%
Tier 1 Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,498,463											$ 1,498,463	$ 1,441,209
Actual, Ratio		10.20%											10.20%	10.70%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 884,307											$ 884,307	$ 805,073
Minimum Required for Capital Adequacy Purposes, Ratio		6.00%											6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,068,538											$ 1,068,538	$ 888,935
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.30%											7.30%	6.60%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual, Capital Amount		$ 1,498,463											$ 1,498,463	$ 1,441,209
Minimum Required for Capital Adequacy Purposes, Capital Amount		10.20%											10.20%	10.70%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 663,230											$ 663,230	$ 603,805
Actual, Ratio		4.50%											4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 847,461											$ 847,461	$ 687,667
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		5.80%											5.80%	5.10%
Leverage Ratio:
Actual, Capital Amount		$ 1,498,463											$ 1,498,463	$ 1,441,209
Actual, Ratio		8.30%											8.30%	9.00%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 720,890											$ 720,890	$ 643,603
Minimum Required for Capital Adequacy Purposes, Ratio		4.00%											4.00%	4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 720,890											$ 720,890	$ 643,603
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%											4.00%	4.00%
Cash dividends from subsidiaries													$ 165,000	$ 110,450	$ 56,860
Chemical Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Risk weighted assets		$ 14,700,000											14,700,000	13,360,000
Total Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,613,087											$ 1,613,087	$ 1,608,980
Actual, Ratio		11.00%											11.00%	12.00%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,176,361											$ 1,176,361	$ 1,068,560
Minimum Required for Capital Adequacy Purposes, Ratio		8.00%											8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,360,167											$ 1,360,167	$ 1,152,041
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.30%											9.30%	8.60%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 1,470,451											$ 1,470,451	$ 1,335,700
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio		10.00%											10.00%	10.00%
Tier 1 Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,513,219											$ 1,513,219	$ 1,522,711
Actual, Ratio		10.30%											10.30%	11.40%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 882,271											$ 882,271	$ 801,420
Minimum Required for Capital Adequacy Purposes, Ratio		6.00%											6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,066,077											$ 1,066,077	$ 884,901
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.30%											7.30%	6.60%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 1,176,361											$ 1,176,361	$ 1,068,560
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio		8.00%											8.00%	8.00%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual, Capital Amount		$ 1,513,219											$ 1,513,219	$ 1,522,711
Minimum Required for Capital Adequacy Purposes, Capital Amount		10.30%											10.30%	11.40%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 661,703											$ 661,703	$ 601,065
Actual, Ratio		4.50%											4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 845,509											$ 845,509	$ 684,546
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		5.80%											5.80%	5.10%
Minimum Required for Capital Adequacy Purposes, Ratio		$ 955,793											$ 955,793	$ 868,205
Required to be Well Capital Under Prompt Corrective Action Regulations, Ratio		6.50%											6.50%	6.50%
Leverage Ratio:
Actual, Capital Amount		$ 1,513,219											$ 1,513,219	$ 1,522,711
Actual, Ratio		8.40%											8.40%	9.50%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 720,043											$ 720,043	$ 641,457
Minimum Required for Capital Adequacy Purposes, Ratio		4.00%											4.00%	4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 720,043											$ 720,043	$ 641,457
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%											4.00%	4.00%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 900,053											$ 900,053	$ 801,822
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio		5.00%											5.00%	5.00%