| **Business Combination (Tables)**                                                               | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
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| **Business Acquisition [Line Items]**                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Schedule of pro forma information from acquisition                                              | The following schedule includes pro forma results for the periods ended December 31, 2014, 2013 and 2012 as if the Community and CFS acquisitions had occurred as of the beginning of the comparable prior annual reporting period. 2014 2013 2012Total revenue (net interest income plus other income)$263,070 $253,668 $266,034Net income$61,572 $39,979 $50,092Net income available to common shareholders$61,572 $37,599 $45,553Earnings per share:     Basic$1.63 $0.98 $1.28Diluted$1.61 $0.97 $1.27                                                                                                                                                                                                                            |
| Merger with Community Bancshares, Inc.                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| **Business Acquisition [Line Items]**                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Schedule of preliminary valuations of the fair value of assets acquired and liabilities assumed | 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. Fair ValueCash and cash equivalents$4,124Interest -bearing time deposits16,526Investment Securities, available for sale76,807Loans145,064Premises and equipment3,610Federal Home Loan Bank stock1,950Interest Receivable767Cash surrender value of life insurance3,266Other real estate owned6,662Taxes, deferred and receivable3,348Other assets167Deposits(228,424)Interest payable(98)Other liabilities(3,014)Net tangible assets acquired$30,755Core deposit intangible4,658Goodwill13,776Purchase price$49,189 |
| Merger with CFS Bancorp, Inc. (Citizens)                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| **Business Acquisition [Line Items]**                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Schedule of preliminary valuations of the fair value of assets acquired and liabilities assumed | 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 acquired$80,756Core deposit intangible7,313Goodwill47,573Purchase price$135,642                                              |

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| - Definition
Tabular disclosure of pro forma results of operations for a material business acquisition or series of individually immaterial business acquisitions that are material in the aggregate.
+ 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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| - Definition
Tabular disclosure of the amounts recognized as of the acquisition date for each major class of assets acquired and liabilities assumed. May include but not limited to the following: (a) acquired receivables; (b) contingencies recognized at the acquisition date; and (c) the fair value of noncontrolling interests in the acquiree.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472
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