Acquisitions Unaudited Pro Forma Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Weighted Average Number of Shares Outstanding, Basic	31,367	28,183	27,492
Weighted Average Number of Shares Outstanding, Diluted	31,588	28,352	27,583
Northwestern Bancorp [Member]
Business Acquisition [Line Items]
Business Acquisition, Pro Forma interest and dividend income	247,748	241,499
Business Acquisition, Pro Forma interest expense	16,221	20,014
Business Acquisition, Pro Forma net interest income	231,527	221,485
Business Acquisition, Pro Forma provision for loan losses	6,825	18,329
Business Acquisition, Pro Forma net interest income after provision for loan losses	224,702	203,156
Business Acquisition, Pro Forma noninterest income	79,229	82,492
Business Acquisition, Pro Forma noninterest expense	201,289	206,025
Business Acquisition, Pro Forma pre-tax net income	102,642	79,623
Business Acquisition, Pro Forma federal income tax expense	32,209	23,777
Business Acquisition, Pro Forma Net Income (Loss)	70,433	55,846
Business Acquisition, Pro Forma Earnings Per Share, Basic	2.25	1.98
Business Acquisition, Pro Forma Earnings Per Share, Diluted	2.23	1.97

Business Acquisition, Pro Forma federal income tax expense

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Business Acquisition, Pro Forma interest and dividend income

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Business Acquisition, Pro Forma interest expense

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Business Acquisition, Pro Forma net interest income

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Business Acquisition, Pro Forma net interest income after provision for loan losses

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Business Acquisition, Pro Forma noninterest expense

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Business Acquisition, Pro Forma noninterest income

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Business Acquisition, Pro Forma pre-tax net income

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Business Acquisition, Pro Forma provision for loan losses

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The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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