Parent Company Only Financial Statements (Statements of Income) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Other income													$ 25,918	$ 11,988	$ 1,907
Interest expense	$ 23,257	$ 21,316	$ 17,185	$ 12,799	$ 11,969	$ 6,753	$ 5,442	$ 5,134	$ 5,153	$ 5,234	$ 3,944	$ 3,450	74,557	29,298	17,781
Operating expenses	100,022	119,539	98,237	104,196	114,302	106,144	59,085	58,887	57,824	58,265	56,785	51,020	421,994	338,418	223,894
Income tax benefit	(61,234)	(10,253)	(23,036)	(12,257)	(18,969)	(2,848)	(10,532)	(9,757)	(10,200)	(9,600)	(9,100)	(8,100)	(106,780)	(42,106)	(37,000)
Net income	$ 9,446	$ 40,459	$ 52,014	$ 47,604	$ 47,168	$ 11,484	$ 25,775	$ 23,605	$ 25,504	$ 24,467	$ 19,024	$ 17,835	149,523	108,032	86,830
Chemical Financial Corporation
Condensed Financial Statements, Captions [Line Items]
Cash dividends from subsidiaries													165,000	110,450	56,860
Other income													921	151	144
Total income													165,921	110,601	57,004
Interest expense													3,076	1,816	761
Operating expenses													23,298	30,589	7,794
Total expenses													26,374	32,405	8,555
Income before income taxes and equity in undistributed net income of subsidiaries													139,547	78,196	48,449
Income tax benefit													12,670	11,378	2,582
Equity in undistributed net (loss) income of subsidiaries													(2,694)	18,458	35,799
Net income													$ 149,523	$ 108,032	$ 86,830