Significant Accounting Policies (Details) (USD $)	3 Months Ended
	Mar. 31, 2015 Bank	Mar. 31, 2014	Dec. 31, 2014	Jun. 24, 2014	Sep. 18, 2013	Apr. 20, 2009	Jan. 31, 2008	Apr. 20, 2015
Business Acquisition [Line Items]
Number of banks through which company operates	1
Number of regional banking units Chemical Bank operates through	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	100,000	100,000
Remaining investment in qualified affordable housing projects	4,800,000	3,200,000	4,900,000
Number of shares authorized to be repurchased							500,000
Stock repurchased (shares)	0
Number of remaining shares authorized to be repurchased (shares)	500,000
Common Stock, Shares, Issued	32,847,063	29,865,841	32,774,420	2,500,000	2,213,750
CommonStockIssuedOverAllotment				375,000	288,750
Gross Value of Common Stock sold in public stock offering				80,500,000
Shares Issued, Price Per Share				$ 28.00	$ 26.00
Issuance of common stock, net of issuance costs				$ 76,200,000	$ 53,900,000
Preferred stock, shares authorized	200,000	200,000	200,000			200,000
Preferred stock, shares issued	0	0	0
Preferred stock, shares outstanding	0
Common Stock, Shares Authorized	45,000,000	45,000,000	45,000,000
Northwestern Bancorp [Member]
Business Acquisition [Line Items]
Number of pools of aggregate acquired loans based upon common risk characteristics	4
OAK [Member]
Business Acquisition [Line Items]
Number of pools of aggregate acquired loans based upon common risk characteristics	14
Shareholder Approved Proposal [Member]
Business Acquisition [Line Items]
Preferred stock, shares authorized								2,000,000
Common Stock, Shares Authorized								60,000,000