Regulatory Capital and Reserve Requirements (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 23, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Leverage Ratio
Cash dividends declared per common share (dollars per share)		$ 0.34	$ 0.28
Subsequent Event
Leverage Ratio
Cash dividends declared per common share (dollars per share)	$ 0.34
Chemical Bank
Regulatory Capital
Risk weighted assets		$ 16,150,000		$ 16,070,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,872,628		$ 1,825,742
Actual Ratio		11.60%		11.40%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,695,932		$ 1,586,719
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		10.50%		9.90%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,615,173		$ 1,606,804
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		10.00%		10.00%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,755,372		$ 1,708,724
Actual Ratio		10.90%		10.60%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,372,897		$ 1,265,358
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		8.50%		7.90%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,292,138		$ 1,285,444
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		$ 1,755,372		$ 1,708,724
Actual Ratio		10.90%		10.60%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,130,621		$ 1,024,338
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.00%		6.40%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,049,862		$ 1,044,423
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		6.50%		6.50%
Leverage Ratio
Actual Amount		$ 1,755,372		$ 1,708,724
Actual Ratio		8.60%		8.60%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 813,893		$ 792,184
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount		$ 1,017,366		$ 990,230
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio		5.00%		5.00%
Corporation
Regulatory Capital
Risk weighted assets		$ 16,180,000		$ 16,100,000
Total Capital to Risk-Weighted Assets
Actual Amount		$ 1,896,830		$ 1,855,922
Actual Ratio		11.70%		11.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,698,868		$ 1,590,323
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		10.50%		9.90%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,763,626		$ 1,723,004
Actual Ratio		10.90%		10.70%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,375,274		$ 1,268,232
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		8.50%		7.90%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual Amount		$ 1,763,626		$ 1,723,004
Actual Ratio		10.90%		10.70%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,132,579		$ 1,026,664
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.00%		6.40%
Leverage Ratio
Actual Amount		$ 1,763,626		$ 1,723,004
Actual Ratio		8.70%		8.70%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 814,944		$ 793,669
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%		4.00%