| **Business Combination (Merger with CFS Bancorp, Inc. (Citizens))** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
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| Merger with CFS Bancorp, Inc. (Citizens)                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| **Business Acquisition [Line Items]**                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Business Combination                                                | BUSINESS COMBINATIONOn November 12, 2013, the Corporation acquired 100 percent of CFS Bancorp, Inc. ("CFS") in an all stock transaction.  CFS was headquartered in Munster, Indiana and had 20 full-service banking centers serving the northwestern Indiana and northeastern Illinois areas.  Pursuant to the merger agreement, the shareholders of CFS received 0.65 percent of the Corporation's common stock for each share of CFS Bancorp common stock held.  The Corporation issued approximately 7.1 million shares of common stock, which was valued at approximately $135.6 million.The Corporation engaged in this transaction with the expectation that it would be accretive and add a new market area with a demographic profile consistent with many of the current Indiana markets served by the Bank.  Goodwill resulted from this transaction due to the expected synergies from combining operations.Under the acquisition method of accounting, the total purchase price is allocated to CFS's net tangible and intangible assets based on their current estimated fair values on the date of the acquisition.   Based on preliminary valuations of the fair value of tangible and intangible assets acquired and liabilities assumed, which are based on assumptions that are subject to change, the purchase price for the CFS acquisition was allocated as follows: Fair ValueCash and cash equivalents$10,992Interest-bearing time deposits213,379Investment securities available for sale15,913Investment securities held to maturity14,372Mortgage loans held for sale189Loans603,114Premises and equipment19,643Federal Home Loan Bank stock6,188Interest receivable1,770Cash surrender value of life insurance36,555Other real estate owned12,857Tax asset, deferred and receivable30,717Other assets111,656Deposits(955,432)Securities sold under repurchase agreements(9,830)Federal Home Loan Bank advances(15,000)Interest payable(294)Other liabilities(16,033)Net tangible assets acquired80,756Core deposit intangible7,313Goodwill47,573Purchase price$135,642Of the total purchase price, $7,313,000 has been allocated to a core deposit intangible that will be amortized over its estimated life of 10 years.  The remaining purchase price has been allocated to goodwill, which is not deductible for tax purposes.Prior to the end of the one year measurement period for finalizing the purchase price allocation, if information becomes available which would indicate adjustments are required to the purchase price allocation, such adjustments will be included in the purchase price allocation retrospectively.The results of operations of CFS have been included in the Corporation's consolidated financial statements since the acquisition date.  The following schedule includes pro forma results for the periods ended December 31, 2013 and 2012 as if the CFS acquisition had occurred as of the beginning of the periods presented.   2013 2012Total revenue (net interest income plus other income)$243,972 $266,034Net income$38,637 $50,092Net income available to common shareholders$36,257 $45,553Earnings per share:   Basic$0.98 $1.28Diluted$0.98 $1.27The pro forma information includes adjustments for interest income on loans and securities acquired, amortization of intangibles arising from the transaction, interest expense on deposits acquired, premises expense for the banking centers acquired and the related income tax effects.  The pro forma information for the year ended 2013 includes $4.9 million of operating revenue from CFS since the acquisition and approximately $9.5 million, net of tax, of non-recurring expenses directly attributable to the CFS acquisition.The pro forma financial information is presented for information purposes only and is not indicative of the results of operations that actually would have been achieved had the acquisition consummated as of that time, nor is it intended to be a projection of future results. |

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| - Definition
The entire disclosure for a business combination (or series of individually immaterial business combinations) completed during the period, including background, timing, and recognized assets and liabilities. The disclosure may include leverage buyout transactions (as applicable).
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