| **Intangible Assets**                                    | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
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| **Goodwill and Intangible Assets Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Intangible Assets                                        | Intangible AssetsBusiness CombinationsUnamortized Intangible Assets (Goodwill)COB conducted an annual goodwill impairment test as of September 30, 2014, and determined there was no impairment to goodwill. The goodwill relates to the Granite acquisition.For intangible assets related to business combinations, the following is a summary of the changes in the balance of unamortized intangible assets (goodwill) during the years ended December 31, 2014, 2013 and 2012: (dollars in thousands) For the Twelve Months EndedDecember 31,  2014 2013 2012Gross balance at beginning of year $4,205 $4,205 $3,905Accumulated impairment balance beginning of year — — —Effect of Granite merger — — 300Impairment — — —Accumulated balance at end of year $4,205 $4,205 $4,205Amortized Intangible AssetsCore Deposit PremiumFor intangible assets related to business combinations, the following is a summary of the gross carrying amount and accumulated amortization of amortized intangible assets and the carrying amount of unamortized intangible assets:(dollars in thousands) As of December 31,  2014 2013Core deposit premium related to whole bank acquisitions:    Carrying amount $13,102 $13,102Accumulated amortization 9,147 7,739Net core deposit premium $3,955 $5,363Amortization of core deposit premium intangibles totaled approximately $1.4 million in 2014, $1.4 million in 2013 and $1.4 million in 2012.The following table presents the estimated amortization expense for intangible assets related to business combinations for each of the five calendar years ending December 31, 2019 and the estimated amount amortizable thereafter. These estimates are subject to change in future periods to the extent management determines it is necessary to make adjustments to the carrying value or estimated useful lives of amortized intangible assets.(dollars in thousands)EstimatedAmortizationExpense2015$1,3852016834201761320186132019510Total$3,955Mortgage Servicing RightsMortgage loans serviced for others are not included in the consolidated balance sheets. The unpaid principal balance of mortgage loans serviced for others amounted to $235.0 million and $190.2 million at December 31, 2014 and 2013, respectively, and the fair value of mortgage servicing rights was $1.7 million and $1.6 million as of the same dates. |

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| - Definition
The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final.  May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.
+ References
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