Regulatory Capital (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 26, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Leverage Ratio
Cash dividends declared per common share (in dollars per share)		$ 0.27	$ 0.26
Subsequent Event
Leverage Ratio
Cash dividends declared per common share (in dollars per share)	$ 0.27
Corporation
Regulatory Capital
Risk weighted assets		$ 13,620,000		$ 13,420,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,547,145		$ 1,543,018
Actual Ratio		11.40%		11.50%
For Capital Adequacy Purposes Amount		$ 1,089,998		$ 1,073,431
For Capital Adequacy Purposes Ratio		8.00%		8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,260,310		$ 1,157,293
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.30%		8.60%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,445,929		$ 1,441,209
Actual Ratio		10.60%		10.70%
For Capital Adequacy Purposes Amount		$ 817,498		$ 805,073
For Capital Adequacy Purposes Ratio		6.00%		6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 987,810		$ 888,935
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.30%		6.60%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,445,929		$ 1,441,209
Actual Ratio		10.60%		10.70%
For Capital Adequacy Purposes Amount		$ 613,124		$ 603,805
For Capital Adequacy Purposes Ratio		4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 783,436		$ 687,667
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		5.80%		5.10%
Leverage Ratio
Actual Amount		$ 1,445,929		$ 1,441,209
Actual Ratio		8.90%		9.00%
For Capital Adequacy Purposes Amount		$ 651,811		$ 643,603
For Capital Adequacy Purposes Ratio		4.00%		4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 651,811		$ 643,603
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%
Chemical Bank
Regulatory Capital
Risk weighted assets		$ 13,580,000		$ 13,360,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,581,814		$ 1,608,980
Actual Ratio		11.70%		12.00%
For Capital Adequacy Purposes Amount		$ 1,086,008		$ 1,068,560
For Capital Adequacy Purposes Ratio		8.00%		8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,255,696		$ 1,152,041
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.30%		8.60%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,357,510		$ 1,335,700
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		10.00%		10.00%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,496,181		$ 1,522,711
Actual Ratio		11.00%		11.40%
For Capital Adequacy Purposes Amount		$ 814,506		$ 801,420
For Capital Adequacy Purposes Ratio		6.00%		6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 984,195		$ 884,901
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.30%		6.60%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,086,008		$ 1,068,560
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		8.00%		8.00%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,496,181		$ 1,522,711
Actual Ratio		11.00%		11.40%
For Capital Adequacy Purposes Amount		$ 610,879		$ 601,065
For Capital Adequacy Purposes Ratio		4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 780,568		$ 684,546
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		5.80%		5.10%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 882,381		$ 868,205
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		6.50%		6.50%
Leverage Ratio
Actual Amount		$ 1,496,181		$ 1,522,711
Actual Ratio		9.20%		9.50%
For Capital Adequacy Purposes Amount		$ 650,356		$ 641,457
For Capital Adequacy Purposes Ratio		4.00%		4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 650,356		$ 641,457
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 812,945		$ 801,822
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		5.00%		5.00%