Parent Company Only Financial Statements (Financial Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Premises and equipment	$ 126,896	$ 145,012
Goodwill	1,134,568	1,133,534
Other assets	709,154	650,973
Total assets	19,280,873	17,355,179
Liabilities and Equity [Abstract]
Other liabilities	181,203	134,637
Total liabilities	16,612,124	14,773,653
Shareholders' equity	2,668,749	2,581,526	$ 1,015,974	$ 797,133
Total liabilities and shareholders' equity	19,280,873	17,355,179
Chemical Financial Corporation
Assets
Cash at subsidiary bank	5,761	16,851
Investment in subsidiaries	2,673,858	2,654,458
Premises and equipment	3,931	4,051
Goodwill	1,092	1,092
Deferred tax asset	7,203	30,715
Other assets	31,459	32,869
Total assets	2,723,304	2,740,036
Liabilities and Equity [Abstract]
Other liabilities	18,877	18,345
Non-revolving line-of-credit	19,963	124,625
Subordinated debentures	15,715	15,540
Total liabilities	54,555	158,510
Shareholders' equity	2,668,749	2,581,526
Total liabilities and shareholders' equity	$ 2,723,304	$ 2,740,036