Significant Accounting Policies (Details)			3 Months Ended
	May. 31, 2015 USD ($)	Apr. 01, 2015 USD ($)	Mar. 31, 2016 USD ($) Bank Pools shares	Mar. 31, 2015 USD ($) shares	Dec. 31, 2015 USD ($) shares	Apr. 20, 2015 shares	Apr. 20, 2009 shares	Jan. 31, 2008 shares
Business Acquisition [Line Items]
Number of banks through which company operates | Bank			1
Number of regional banking units Chemical Bank operates through | Bank			4
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
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	$ 0
Expense related to qualified affordable housing projects			600,000	100,000
Remaining investment in qualified affordable housing projects			21,100,000	4,800,000	21,700,000
Equity Method Investments			1,500,000	0	1,700,000
Variable Interest Entity, Reporting Entity Involvement, Unfunded Obligation, Amount			4,200,000		8,000,000
Impairment on tax credit projects			$ 0	$ 0	$ 0
Number of shares authorized to be repurchased | shares								500,000
Stock repurchased (shares) | shares			0
Number of remaining shares authorized to be repurchased (shares) | shares			500,000
Preferred stock, shares authorized | shares			2,000,000	200,000	2,000,000	2,000,000	200,000
Preferred stock, shares issued | shares			0	0	0
Preferred stock, shares outstanding | shares			0
Common Stock, Shares, Issued | shares			38,248,489	32,847,063	38,167,861
Common Stock, Shares Authorized | shares			60,000,000	45,000,000	60,000,000	60,000,000
Goodwill			$ 286,867,000	$ 180,128,000	$ 287,393,000
Lake Michigan Financial Corp [Member]
Business Acquisition [Line Items]
Number of pools of aggregate acquired loans based upon common risk characteristics | Pools			2
Monarch Community Bancorp [Member]
Business Acquisition [Line Items]
Number of pools of aggregate acquired loans based upon common risk characteristics | Pools			2
Northwestern Bancorp [Member]
Business Acquisition [Line Items]
Number of pools of aggregate acquired loans based upon common risk characteristics | Pools			4
OAK [Member]
Business Acquisition [Line Items]
Number of pools of aggregate acquired loans based upon common risk characteristics | Pools			14
Monarch Community Bancorp [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred		$ 27,200,000
Goodwill		$ 5,300,000
Lake Michigan Financial Corp [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred	$ 187,400,000
Business Combination, Consideration Transferred, Value of Stock and Options Issued	132,900,000
Business Combination, Consideration Transferred, Cash Paid for Outstanding Stock	54,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	8,600,000
Goodwill	$ 101,000,000