Regulatory Capital (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 24, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Leverage Ratio
Cash dividends declared per common share (dollars per share)		$ 0.28	$ 0.27
Subsequent Event
Leverage Ratio
Cash dividends declared per common share (dollars per share)	$ 0.28
Chemical Bank
Regulatory Capital
Risk weighted assets		$ 14,800,000		$ 14,700,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,655,547		$ 1,613,087
Actual Ratio		11.20%		11.00%
For Capital Adequacy Purposes Amount		$ 1,184,378		$ 1,176,361
For Capital Adequacy Purposes Ratio		8.00%		8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,461,966		$ 1,360,167
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.90%		9.30%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,480,472		$ 1,470,451
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		10.00%		10.00%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,552,763		$ 1,513,219
Actual Ratio		10.50%		10.30%
For Capital Adequacy Purposes Amount		$ 888,283		$ 882,271
For Capital Adequacy Purposes Ratio		6.00%		6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,165,872		$ 1,066,077
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.90%		7.30%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,184,378		$ 1,176,361
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		$ 1,552,763		$ 1,513,219
Actual Ratio		10.50%		10.30%
For Capital Adequacy Purposes Amount		$ 666,213		$ 661,703
For Capital Adequacy Purposes Ratio		4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 943,801		$ 854,509
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		6.40%		5.80%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 962,307		$ 955,793
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		6.50%		6.50%
Leverage Ratio
Actual Amount		$ 1,552,763		$ 1,513,219
Actual Ratio		8.50%		8.40%
For Capital Adequacy Purposes Amount		$ 730,292		$ 720,043
For Capital Adequacy Purposes Ratio		4.00%		4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 730,292		$ 720,043
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 912,865		$ 900,053
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		5.00%		5.00%
Corporation
Regulatory Capital
Risk weighted assets		$ 14,840,000		$ 14,740,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,662,561		$ 1,614,046
Actual Ratio		11.20%		11.00%
For Capital Adequacy Purposes Amount		$ 1,186,869		$ 1,179,076
For Capital Adequacy Purposes Ratio		8.00%		8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,465,041		$ 1,363,307
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.90%		9.30%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,544,016		$ 1,498,463
Actual Ratio		10.40%		10.20%
For Capital Adequacy Purposes Amount		$ 890,152		$ 884,307
For Capital Adequacy Purposes Ratio		6.00%		6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,168,324		$ 1,068,538
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.90%		7.30%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,544,016		$ 1,498,463
Actual Ratio		10.40%		10.20%
For Capital Adequacy Purposes Amount		$ 667,614		$ 663,230
For Capital Adequacy Purposes Ratio		4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 945,786		$ 847,461
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		6.40%		5.80%
Leverage Ratio
Actual Amount		$ 1,544,016		$ 1,498,463
Actual Ratio		8.40%		8.30%
For Capital Adequacy Purposes Amount		$ 731,240		$ 720,890
For Capital Adequacy Purposes Ratio		4.00%		4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 731,240		$ 720,890
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%