Significant Accounting Policies (Details)			3 Months Ended		9 Months Ended
	May. 31, 2015 USD ($)	Apr. 01, 2015 USD ($)	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($) shares	Sep. 30, 2015 USD ($) Bank Pools shares	Sep. 30, 2014 USD ($) shares	Apr. 20, 2015 shares	Mar. 31, 2015 shares	Dec. 31, 2014 USD ($) shares	Jun. 24, 2014 USD ($) $ / shares shares	Sep. 18, 2013 USD ($) $ / shares shares	Apr. 20, 2009 shares	Jan. 31, 2008 shares
Business Acquisition [Line Items]
Number of banks through which company operates | Bank					3
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	$ 0			$ 0
Expense related to qualified affordable housing projects			600,000	100,000	800,000	300,000
Remaining investment in qualified affordable housing projects			$ 20,500,000	$ 3,000,000	$ 20,500,000	$ 3,000,000			$ 4,900,000
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		500,000
Common Stock, Shares, Issued | shares			38,130,965	32,762,591	38,130,965	32,762,591			32,774,420	2,500,000	2,213,750
CommonStockIssuedOverAllotment | shares										375,000	288,750
Gross Value of Common Stock sold in public stock offering										$ 80,500,000
Shares Issued, Price Per Share | $ / shares										$ 28.00	$ 26.00
Issuance of common stock, net of issuance costs										$ 76,200,000	$ 53,900,000
Preferred stock, shares authorized | shares			2,000,000	200,000	2,000,000	200,000			200,000			200,000
Preferred stock, shares issued | shares			0	0	0	0			0
Preferred stock, shares outstanding | shares			0		0
Common Stock, Shares Authorized | shares			60,000,000	45,000,000	60,000,000	45,000,000		45,000,000	45,000,000
Equity Method Investments			$ 3,200,000	$ 0	$ 3,200,000	$ 0			$ 0
Variable Interest Entity, Reporting Entity Involvement, Unfunded Obligation, Amount			8,700,000		8,700,000
Goodwill			286,454,000	120,164,000	286,454,000	120,164,000			180,128,000
Impairment on tax credit projects			$ 0	$ 0	$ 0	$ 0			$ 0
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
Shareholder Approved Proposal [Member]
Business Acquisition [Line Items]
Preferred stock, shares authorized | shares							2,000,000
Common Stock, Shares Authorized | shares							60,000,000
Monarch Community Bancorp [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred		$ 27,191,000
Business Combination, Consideration Transferred, Cash Paid for Outstanding Stock		203,000
Goodwill		5,314,000
Purchase Accounting Fair Value Adjustment, Core Deposit Intangible Asset		$ 1,930,000
Lake Michigan Financial Corp [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred	$ 187,394,000
Business Combination, Consideration Transferred, Value of Stock and Options Issued	132,900,000
Business Combination, Consideration Transferred, Cash Paid for Outstanding Stock	54,478,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	8,600,000
Goodwill	100,878,000
Purchase Accounting Fair Value Adjustment, Core Deposit Intangible Asset	$ 8,000,000