Parent Company Only Financial Statements (Statements of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	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
Cash Flows From Operating Activities
Net income	$ 73,039	$ 70,397	$ 68,988	$ 71,596	$ 9,446	$ 40,459	$ 52,014	$ 47,604	$ 47,168	$ 11,484	$ 25,775	$ 23,605	$ 284,020	$ 149,523	$ 108,032
Share-based compensation expense													8,578	17,346	13,452
Depreciation of premises and equipment													17,026	17,722	13,270
Net cash provided by operating activities													324,401	274,866	280,501
Cash Flows From Investing Activities
Purchases of premises and equipment, net													(13,572)	(12,852)	(18,098)
Net cash used in investing activities													(2,199,328)	(1,982,936)	(542,869)
Cash Flows From Financing Activities
Cash dividends paid													(88,954)	(78,498)	(49,389)
Repayments of other borrowings													(147,000)	(224,850)	(351,018)
Proceeds from issuance of other borrowings													200,000	0	375,000
Proceeds from directors' stock purchase plan and exercise of stock options													3,115	3,991	1,572
Cash paid for payroll taxes upon conversion of restricted stock units													(5,316)	(27,854)	(1,065)
Net cash provided by financing activities													1,914,775	1,689,659	497,981
Net increase (decrease) in cash and cash equivalents													39,848	(18,411)	235,613
Cash and cash equivalents at beginning of year				455,991				474,402				238,789	455,991	474,402	238,789
Cash and cash equivalents at end of year	495,839				455,991				474,402				495,839	455,991	474,402
Business combinations:
Fair value of tangible assets acquired (noncash)													0	420	6,371,781
Liabilities assumed													0	1,454	6,100,901
Common stock and stock options issued													0	0	1,504,811
Chemical Financial Corporation
Cash Flows From Operating Activities
Net income													284,020	149,523	108,032
Share-based compensation expense													8,578	17,346	13,452
Depreciation of premises and equipment													435	408	355
Equity in undistributed net (income) loss of subsidiaries													(180,902)	2,694	(18,458)
Net decrease in other assets													4,302	1,410	9,752
Net increase (decrease) in other liabilities													537	25,178	(11,830)
Net cash provided by operating activities													116,970	196,559	101,303
Cash Flows From Investing Activities
Cash acquired, net of cash paid, in business combinations													0	0	(107,622)
Purchases of premises and equipment, net													(99)	(288)	(774)
Net cash used in investing activities													(99)	(288)	(108,396)
Cash Flows From Financing Activities
Cash dividends paid													(88,954)	(78,498)	(49,389)
Proceeds from issuance of common stock, net of issuance costs													0	0	0
Repayment of subordinated debt obligations													0	0	(18,558)
Repayments of other borrowings													(20,000)	(105,000)	(62,500)
Proceeds from issuance of other borrowings													0	0	125,000
Proceeds from directors' stock purchase plan and exercise of stock options													3,115	3,991	1,572
Cash paid for payroll taxes upon conversion of restricted stock units													(5,316)	(27,854)	(1,065)
Net cash provided by financing activities													(111,155)	(207,361)	(4,940)
Net increase (decrease) in cash and cash equivalents													5,716	(11,090)	(12,033)
Cash and cash equivalents at beginning of year				$ 5,761				$ 16,851				$ 28,884	5,761	16,851	28,884
Cash and cash equivalents at end of year	$ 11,477				$ 5,761				$ 16,851				11,477	5,761	16,851
Business combinations:
Fair value of tangible assets acquired (noncash)													0	0	46,898
Liabilities assumed													0	0	58,309
Common stock and stock options issued													$ 0	$ 0	$ 1,504,811