Regulatory Capital and Reserve Requirements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Leverage Ratio
Cash dividends declared per common share (dollars per share)	$ 0.34	$ 0.28	$ 0.68	$ 0.56
Chemical Bank
Regulatory Capital
Risk weighted assets	$ 16,990,000		$ 16,990,000		$ 16,070,000
Total Capital to Risk-Weighted Assets
Actual Amount	$ 1,928,735		$ 1,928,735		$ 1,825,742
Actual Ratio	11.30%		11.30%		11.40%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 1,784,388		$ 1,784,388		$ 1,586,719
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio	10.50%		10.50%		9.90%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,699,417		$ 1,699,417		$ 1,606,804
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	$ 1,806,734		$ 1,806,734		$ 1,708,724
Actual Ratio	10.60%		10.60%		10.60%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 1,444,504		$ 1,444,504		$ 1,265,358
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio	8.50%		8.50%		7.90%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,359,534		$ 1,359,534		$ 1,285,444
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	$ 1,806,734		$ 1,806,734		$ 1,708,724
Actual Ratio	10.60%		10.60%		10.60%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 1,189,592		$ 1,189,592		$ 1,024,338
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio	7.00%		7.00%		6.40%
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,104,621		$ 1,104,621		$ 1,044,423
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	6.50%		6.50%		6.50%
Leverage Ratio
Actual Amount	$ 1,806,734		$ 1,806,734		$ 1,708,724
Actual Ratio	8.70%		8.70%		8.60%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 832,372		$ 832,372		$ 792,184
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	$ 1,040,465		$ 1,040,465		$ 990,230
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	5.00%		5.00%		5.00%
Corporation
Regulatory Capital
Risk weighted assets	$ 17,020,000		$ 17,020,000		$ 16,100,000
Total Capital to Risk-Weighted Assets
Actual Amount	$ 1,951,444		$ 1,951,444		$ 1,855,922
Actual Ratio	11.50%		11.50%		11.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 1,787,527		$ 1,787,527		$ 1,590,323
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio	10.50%		10.50%		9.90%
Tier 1 Capital to Risk-Weighted Assets
Actual Amount	$ 1,813,446		$ 1,813,446		$ 1,723,004
Actual Ratio	10.70%		10.70%		10.70%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 1,447,046		$ 1,447,046		$ 1,268,232
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio	8.50%		8.50%		7.90%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual Amount	$ 1,813,446		$ 1,813,446		$ 1,723,004
Actual Ratio	10.70%		10.70%		10.70%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 1,191,685		$ 1,191,685		$ 1,026,664
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio	7.00%		7.00%		6.40%
Leverage Ratio
Actual Amount	$ 1,813,446		$ 1,813,446		$ 1,723,004
Actual Ratio	8.70%		8.70%		8.70%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount	$ 833,418		$ 833,418		$ 793,669
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio	4.00%		4.00%		4.00%