Regulatory Capital (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Oct. 18, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Leverage Ratio
Cash dividends declared per common share (dollars per share)		$ 0.28	$ 0.27	$ 0.82	$ 0.79000
Subsequent Event
Leverage Ratio
Cash dividends declared per common share (dollars per share)	$ 0.28
Chemical Bank
Regulatory Capital
Risk weighted assets		$ 14,480,000		$ 14,480,000		$ 13,360,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,619,878		$ 1,619,878		$ 1,608,980
Actual Ratio		11.20%		11.20%		12.00%
For Capital Adequacy Purposes Amount		$ 1,158,070		$ 1,158,070		$ 1,068,560
For Capital Adequacy Purposes Ratio		8.00%		8.00%		8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,339,019		$ 1,339,019		$ 1,152,041
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.30%		9.30%		8.60%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,447,588		$ 1,447,588		$ 1,335,700
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		10.00%		10.00%		10.00%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,526,212		$ 1,526,212		$ 1,522,711
Actual Ratio		10.50%		10.50%		11.40%
For Capital Adequacy Purposes Amount		$ 868,553		$ 868,553		$ 801,420
For Capital Adequacy Purposes Ratio		6.00%		6.00%		6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,049,501		$ 1,049,501		$ 884,901
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.30%		7.30%		6.60%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,158,070		$ 1,158,070		$ 1,068,560
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		8.00%		8.00%		8.00%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,526,212		$ 1,526,212		$ 1,522,711
Actual Ratio		10.50%		10.50%		11.40%
For Capital Adequacy Purposes Amount		$ 651,414		$ 651,414		$ 601,065
For Capital Adequacy Purposes Ratio		4.50%		4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 832,363		$ 832,363		$ 684,546
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		5.80%		5.80%		5.10%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 940,932		$ 940,932		$ 868,205
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		6.50%		6.50%		6.50%
Leverage Ratio
Actual Amount		$ 1,526,212		$ 1,526,212		$ 1,522,711
Actual Ratio		8.60%		8.60%		9.50%
For Capital Adequacy Purposes Amount		$ 706,323		$ 706,323		$ 641,457
For Capital Adequacy Purposes Ratio		4.00%		4.00%		4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 706,323		$ 706,323		$ 641,457
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%		4.00%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 882,903		$ 882,903		$ 801,822
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		5.00%		5.00%		5.00%
Corporation
Regulatory Capital
Risk weighted assets		$ 14,510,000		$ 14,510,000		$ 13,420,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,626,898		$ 1,626,898		$ 1,543,018
Actual Ratio		11.20%		11.20%		11.50%
For Capital Adequacy Purposes Amount		$ 1,160,602		$ 1,160,602		$ 1,073,431
For Capital Adequacy Purposes Ratio		8.00%		8.00%		8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,341,946		$ 1,341,946		$ 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 Amount		$ 1,517,561		$ 1,517,561		$ 1,441,209
Actual Ratio		10.50%		10.50%		10.70%
For Capital Adequacy Purposes Amount		$ 870,451		$ 870,451		$ 805,073
For Capital Adequacy Purposes Ratio		6.00%		6.00%		6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,051,795		$ 1,051,795		$ 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 Amount		$ 1,517,561		$ 1,517,561		$ 1,441,209
Actual Ratio		10.50%		10.50%		10.70%
For Capital Adequacy Purposes Amount		$ 652,839		$ 652,839		$ 603,805
For Capital Adequacy Purposes Ratio		4.50%		4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 834,183		$ 834,183		$ 687,667
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		5.80%		5.80%		5.10%
Leverage Ratio
Actual Amount		$ 1,517,561		$ 1,517,561		$ 1,441,209
Actual Ratio		8.60%		8.60%		9.00%
For Capital Adequacy Purposes Amount		$ 707,330		$ 707,330		$ 643,603
For Capital Adequacy Purposes Ratio		4.00%		4.00%		4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 707,330		$ 707,330		$ 643,603
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%		4.00%