Significant Accounting Policies (Details)					3 Months Ended		9 Months Ended
	Nov. 04, 2016 USD ($)	Oct. 28, 2016 USD ($)	Aug. 31, 2016 USD ($)	Feb. 22, 2016 claim	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) pool Bank shares	Sep. 30, 2015 USD ($)	Jul. 19, 2016 shares	Jul. 18, 2016 shares	Dec. 31, 2015 USD ($) shares	Apr. 20, 2015 shares	Apr. 20, 2009 shares	Jan. 31, 2008 shares
Business Acquisition
Number of banks through which company operates (in bank) | Bank							2
Number of regional banking units Chemical Bank operates through (in bank) | Bank							7
Goodwill					$ 1,137,166,000		$ 1,137,166,000				$ 287,393,000
Number of minimum days when principal or interest is past due is placed in nonaccrual category							90 days
Number of minimum days in real estate residential loan when principal or interest is past due is placed in nonaccrual category							120 days
Time period in which payments receive consistently and loans are returned to accrual status							6 months
Time period sustained for payment history of principal and interest payments to move them to performing status							6 months
Time period of real estate residential troubled debt restructurings reducing borrower's monthly payments by decreasing the interest rate charged on the loan							24 months
Minimum number of days past due as to interest or principal payments in accruing loans moves to nonperforming loans							90 days
Minimum amount of loan reviewed for detailed credit quality							$ 250,000.00
Time period for historical loan loss experience							5 years
Minimum percentage of tax benefit is recognized for the amount likely to be realized							50.00%
Reserve for contingent income tax liabilities recorded					0		$ 0
Expense related to qualified affordable housing projects					400,000	$ 600,000	1,600,000	$ 800,000
Remaining investment in qualified affordable housing projects					30,300,000		30,300,000				21,700,000
Equity method investments					10,200,000		10,200,000				1,700,000
Remaining unfunded equity contributions					16,800,000		16,800,000				8,000,000
Impairment on tax credit projects					$ 0		$ 0				$ 0
Number of shares authorized to be repurchased (in shares) | shares														500,000
Stock repurchased (in shares) | shares							0
Number of remaining shares authorized to be repurchased (in shares) | shares					500,000		500,000
Preferred stock, shares authorized (in shares) | shares					2,000,000		2,000,000				2,000,000	2,000,000	200,000
Preferred stock, shares issued (in shares) | shares					0		0				0
Preferred stock, shares outstanding (in shares) | shares					0		0
Common stock, shares authorized (in shares) | shares					100,000,000		100,000,000		100,000,000	60,000,000	60,000,000
Subsequent Event [Member] | Single branch office in Chicago [Member] | Completed sale [Member]
Business Acquisition
Deposits assumed in sale		$ 48,900,000
Loans purchased in sale		233,600,000
Premium in sale transaction		$ 6,500,000
Subsequent Event [Member] | Single branch office in Las Vegas [Member] | Completed sale [Member]
Business Acquisition
Deposits assumed in sale	$ 88,100,000
Loans purchased in sale	100,000
Premium in sale transaction	$ 1,000,000
Pending Litigation [Member] | In re Talmer Bancorp Shareholder Litigation [Member]
Business Acquisition
Number of putative class action and derivative complaints filed (in claim) | claim				2
Talmer [Member]
Business Acquisition
Total consideration			$ 1,612,449,000
Stock consideration			1,504,811,000
Cash consideration			107,638,000
Goodwill			850,299,000
Intangibles acquired			12,399,000
Deposits acquired			$ 5,290,000,000
Talmer [Member]
Business Acquisition
Number of pools of aggregate acquired loans based upon common risk characteristics (in pool) | pool							5
Lake Michigan Financial Corporation [Member]
Business Acquisition
Number of pools of aggregate acquired loans based upon common risk characteristics (in pool) | pool							2
Monarch Community Bancorp [Member]
Business Acquisition
Number of pools of aggregate acquired loans based upon common risk characteristics (in pool) | pool							2
Northwestern Bancorp [Member]
Business Acquisition
Number of pools of aggregate acquired loans based upon common risk characteristics (in pool) | pool							4
O.A.K. Financial Corporation [Member]
Business Acquisition
Number of pools of aggregate acquired loans based upon common risk characteristics (in pool) | pool							14