Retirement Plans (Fair Value of Plan Assets by Fair Value Level) (Details) - Pension Plan - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 145,901,000	$ 132,751,000	$ 126,930,000
Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	145,901,000	132,751,000
Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	145,353,000	130,442,000
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	548,000	2,309,000
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,238,000	2,825,000
Cash | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,238,000	2,825,000
Cash | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. large- and mid-cap stocks
Defined Benefit Plan Disclosure [Line Items]
Minimum market capitalization for investments	3,000,000,000
U.S. large- and mid-cap stocks | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68,909,000	55,389,000
U.S. large- and mid-cap stocks | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68,909,000	55,389,000
U.S. large- and mid-cap stocks | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. large- and mid-cap stocks | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. small-cap mutual funds | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,377,000	5,687,000
U.S. small-cap mutual funds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,377,000	5,687,000
U.S. small-cap mutual funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. small-cap mutual funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International large-cap mutual funds | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18,341,000	13,701,000
International large-cap mutual funds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18,341,000	13,701,000
International large-cap mutual funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International large-cap mutual funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Emerging markets mutual funds | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,363,000	5,147,000
Emerging markets mutual funds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,363,000	5,147,000
Emerging markets mutual funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Emerging markets mutual funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Chemical Financial Corporation common stock | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		11,412,000
Chemical Financial Corporation common stock | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		11,412,000
Chemical Financial Corporation common stock | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Chemical Financial Corporation common stock | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
U.S. Treasury and government sponsored agency bonds and notes | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	548,000	806,000
U.S. Treasury and government sponsored agency bonds and notes | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Treasury and government sponsored agency bonds and notes | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	548,000	806,000
U.S. Treasury and government sponsored agency bonds and notes | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate bonds | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,503,000
Corporate bonds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Corporate bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,503,000
Corporate bonds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Mutual funds | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40,046,000	36,220,000
Mutual funds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40,046,000	36,220,000
Mutual funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Mutual funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other | Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	79,000	61,000
Other | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	79,000	61,000
Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0