Parent Company Only Financial Statements (Statements of Cash Flows) (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
Cash Flows From Operating Activities
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
Share-based compensation expense													17,346	13,452	3,478
Depreciation of premises and equipment													17,722	13,270	11,028
Net cash from operating activities													275,009	280,578	103,049
Cash Flows From Investing Activities
Purchases of premises and equipment, net													(12,852)	(18,098)	(8,527)
Net cash used in investing activities													(1,983,079)	(542,946)	(408,784)
Cash Flows From Financing Activities
Cash dividends paid													(78,498)	(49,389)	(36,918)
Repayments of other borrowings													(224,850)	(351,018)	(6,000)
Proceeds from issuance of other borrowings													0	375,000	125,000
Proceeds from directors' stock purchase plan and exercise of stock options													3,991	1,572	2,473
Cash paid for payroll taxes upon conversion of restricted stock units													(27,854)	(1,065)	(571)
Net cash from financing activities													1,689,659	497,981	361,504
Net increase (decrease) in cash and cash equivalents													(18,411)	235,613	55,769
Cash and cash equivalents at beginning of year				474,402				238,789				183,020	474,402	238,789	183,020
Cash and cash equivalents at end of year	455,991				474,402				238,789				455,991	474,402	238,789
Business combinations:
Fair value of tangible assets acquired (noncash)													420	6,371,781	1,282,420
Liabilities assumed													1,454	6,100,901	1,258,051
Common stock and stock options issued													0	1,504,811	159,904
Chemical Financial Corporation
Cash Flows From Operating Activities
Net Income													149,523	108,032	86,830
Share-based compensation expense													17,346	13,452	3,478
Depreciation of premises and equipment													408	355	318
Equity in undistributed net loss (income) of subsidiaries													2,694	(18,458)	(35,799)
Net decrease in other assets													1,410	9,752	2,873
Net increase (decrease) in other liabilities													25,178	(11,830)	(19,033)
Net cash from operating activities													196,559	101,303	38,667
Cash Flows From Investing Activities
Cash acquired, net of cash paid, in business combinations													0	(107,622)	(45,267)
Purchases of premises and equipment, net													(288)	(774)	(320)
Net cash used in investing activities													(288)	(108,396)	(45,587)
Cash Flows From Financing Activities
Cash dividends paid													(78,498)	(49,389)	(36,918)
Proceeds from issuance of common stock, net of issuance costs													0	0	0
Repayment of subordinated debt obligations													0	(18,558)	0
Repayments of other borrowings													(105,000)	(62,500)	0
Proceeds from issuance of other borrowings													0	125,000	25,000
Proceeds from directors' stock purchase plan and exercise of stock options													3,991	1,572	2,473
Cash paid for payroll taxes upon conversion of restricted stock units													(27,854)	(1,065)	(571)
Net cash from financing activities													(207,361)	(4,940)	(10,016)
Net increase (decrease) in cash and cash equivalents													(11,090)	(12,033)	(16,936)
Cash and cash equivalents at beginning of year				$ 16,851				$ 28,884				$ 45,820	16,851	28,884	45,820
Cash and cash equivalents at end of year	$ 5,761				$ 16,851				$ 28,884				5,761	16,851	28,884
Business combinations:
Fair value of tangible assets acquired (noncash)													0	46,898	10,304
Liabilities assumed													0	58,309	42,019
Common stock and stock options issued													$ 0	$ 1,504,811	$ 159,904