Regulatory Capital and Reserve Requirements (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended												12 Months Ended
	Jan. 22, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum average regulatory cash reserve balance		$ 57,000				$ 71,700								$ 57,000	$ 71,700
Leverage Ratio:
Cash dividends declared per share (in dollars per share)		$ 0.34	$ 0.34	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.26	$ 0.26	$ 1.24	$ 1.10	$ 1.06
Subsequent Event
Leverage Ratio:
Cash dividends declared per share (in dollars per share)	$ 0.34
Chemical Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Risk weighted assets		$ 16,100,000				$ 14,740,000								$ 16,100,000	$ 14,740,000
Total Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,855,922				$ 1,614,046								$ 1,855,922	$ 1,614,046
Actual, Ratio		11.50%				11.00%								11.50%	11.00%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,288,363				$ 1,179,076								$ 1,288,363	$ 1,179,076
Minimum Required for Capital Adequacy Purposes, Ratio		8.00%				8.00%								8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,590,323				$ 1,363,307								$ 1,590,323	$ 1,363,307
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.90%				9.30%								9.90%	9.30%
Tier 1 Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,723,004				$ 1,498,463								$ 1,723,004	$ 1,498,463
Actual, Ratio		10.70%				10.20%								10.70%	10.20%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 966,272				$ 884,307								$ 966,272	$ 884,307
Minimum Required for Capital Adequacy Purposes, Ratio		6.00%				6.00%								6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,268,232				$ 1,068,538								$ 1,268,232	$ 1,068,538
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.90%				7.30%								7.90%	7.30%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual, Capital Amount		$ 1,723,004				$ 1,498,463								$ 1,723,004	$ 1,498,463
Minimum Required for Capital Adequacy Purposes, Capital Amount		10.70%				10.20%								10.70%	10.20%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 724,704				$ 663,230								$ 724,704	$ 663,230
Actual, Ratio		4.50%				4.50%								4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,026,664				$ 847,461								$ 1,026,664	$ 847,461
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		6.40%				5.80%								6.40%	5.80%
Leverage Ratio:
Actual, Capital Amount		$ 1,723,004				$ 1,498,463								$ 1,723,004	$ 1,498,463
Actual, Ratio		8.70%				8.30%								8.70%	8.30%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 793,669				$ 720,890								$ 793,669	$ 720,890
Minimum Required for Capital Adequacy Purposes, Ratio		4.00%				4.00%								4.00%	4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 793,669				$ 720,890								$ 793,669	$ 720,890
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%				4.00%								4.00%	4.00%
Cash dividends from subsidiaries														$ 114,300	$ 165,000	$ 110,450
Chemical Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Risk weighted assets		$ 16,070,000				$ 14,700,000								16,070,000	14,700,000
Total Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,825,742				$ 1,613,087								$ 1,825,742	$ 1,613,087
Actual, Ratio		11.40%				11.00%								11.40%	11.00%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,285,444				$ 1,176,361								$ 1,285,444	$ 1,176,361
Minimum Required for Capital Adequacy Purposes, Ratio		8.00%				8.00%								8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,586,719				$ 1,360,167								$ 1,586,719	$ 1,360,167
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		9.90%				9.30%								9.90%	9.30%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 1,606,804				$ 1,470,451								$ 1,606,804	$ 1,470,451
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio		10.00%				10.00%								10.00%	10.00%
Tier 1 Capital to Risk-Weighted Assets:
Actual, Capital Amount		$ 1,708,724				$ 1,513,219								$ 1,708,724	$ 1,513,219
Actual, Ratio		10.60%				10.30%								10.60%	10.30%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 964,083				$ 882,271								$ 964,083	$ 882,271
Minimum Required for Capital Adequacy Purposes, Ratio		6.00%				6.00%								6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,265,358				$ 1,066,077								$ 1,265,358	$ 1,066,077
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		7.90%				7.30%								7.90%	7.30%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 1,285,444				$ 1,176,361								$ 1,285,444	$ 1,176,361
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio		8.00%				8.00%								8.00%	8.00%
Common Equity Tier 1 Capital to Risk-Weighted Assets
Actual, Capital Amount		$ 1,708,724				$ 1,513,219								$ 1,708,724	$ 1,513,219
Minimum Required for Capital Adequacy Purposes, Capital Amount		10.60%				10.30%								10.60%	10.30%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 723,062				$ 661,703								$ 723,062	$ 661,703
Actual, Ratio		4.50%				4.50%								4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 1,024,338				$ 845,509								$ 1,024,338	$ 845,509
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		6.40%				5.80%								6.40%	5.80%
Minimum Required for Capital Adequacy Purposes, Ratio		$ 1,044,423				$ 955,793								$ 1,044,423	$ 955,793
Required to be Well Capital Under Prompt Corrective Action Regulations, Ratio		6.50%				6.50%								6.50%	6.50%
Leverage Ratio:
Actual, Capital Amount		$ 1,708,724				$ 1,513,219								$ 1,708,724	$ 1,513,219
Actual, Ratio		8.60%				8.40%								8.60%	8.40%
Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 792,184				$ 720,043								$ 792,184	$ 720,043
Minimum Required for Capital Adequacy Purposes, Ratio		4.00%				4.00%								4.00%	4.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer Amount		$ 792,184				$ 720,043								$ 792,184	$ 720,043
For Capital Adequacy Purposes Plus Capital Conservation Buffer Ratio		4.00%				4.00%								4.00%	4.00%
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount		$ 990,230				$ 900,053								$ 990,230	$ 900,053
Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio		5.00%				5.00%								5.00%	5.00%