| **Business Combination - Pro Forma Information (Details) (Merger with CFS Bancorp, Inc. (Citizens), USD $)In Thousands, except Per Share data, unless otherwise specified** | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
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| Merger with CFS Bancorp, Inc. (Citizens)                                                                                                                                    |            |   |
| **Business Acquisition [Line Items]**                                                                                                                                       |            |   |
| Total revenue (net interest income plus other income)                                                                                                                       | $ 243,972       | $ 266,034 |
| Net income                                                                                                                                                                  | 38,637          | 50,092 |
| Net income available to common shareholders                                                                                                                                 | $ 36,257        | $ 45,553 |
| **Earnings per share:**                                                                                                                                                     |            |   |
| Basic (in dollars per share)                                                                                                                                                | $ 0.98          | $ 1.28 |
| Diluted (in dollars per share)                                                                                                                                              | $ 0.98          | $ 1.27 |

| X                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Acquisition, Pro Forma Net Income Loss, Attributable to Parent
+ References
No definition available.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Acquisition, Pro Forma Revenue, Net Interest Income Plus Other Income
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463
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