|**Goodwill**                                             | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|**Goodwill and Intangible Assets Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Goodwill                                                 | GOODWILLGoodwill is recorded on the acquisition date of an entity.  During the measurement period, the Corporation may record subsequent adjustments to goodwill for provisional amounts recorded at the acquisition date.   The IAB acquisition on July, 14, 2017 resulted in$153,636,000of goodwill. The Arlington Bank acquisition on May 19, 2017 resulted in$47,719,000of goodwill, which includes a reduction of$469,000. This reduction was recorded in the third quarter of 2017 as a measurement period adjustment.  Details regarding the IAB and Arlington Bank acquisitions are discussed in NOTE 2. ACQUISITIONS of these Notes to Consolidated Financial Statements. There have been no changes in goodwill sinceDecember 31, 2017, resulting in a goodwill balance of$445,355,000as ofDecember 31, 2018.Noimpairment loss was recorded in 2018 or 2017.  The Corporation tested goodwill for impairment during 2018 and 2017.  In both valuations, the fair value exceeded the Corporation’s carrying value; therefore, it was concluded goodwill is not impaired. For additional details related to impairment testing, see the “GOODWILL” section of Management’s Discussion and Analysis of Financial Condition and Results of Operations included as Item 7 of this Annual Report on Form 10-K.2017Balance, January 1$244,000Goodwill acquired201,824Measurement period adjustment$(469)Balance, December 31$445,355
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|Balance, January 1                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Goodwill acquired                                        | 201,824                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Measurement period adjustment                            | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Balance, December 31                                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

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