|**Fair Value Measurements and Disclosures Fair value of the portion of long-term debt related to senior and convertible notes (Tables)** | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|**Fair Value Disclosures [Abstract]**                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block]                                             | The portion of our long-term debt related to our revolving credit facility approximates fair value due to the variable nature of related interest rates.  We have not elected to account for the portion of our debt related to our senior notes under the fair value option; however, we have determined an estimate of the fair values based on measurements of trading activity and broker and/or dealer quotes, respectively, which are published market prices, and therefore are Level 2 inputs.  The table below presents these estimates of the fair value of the portion of our long-term debt related to our senior notes and convertible notes as ofJune 30, 2017.Estimated Fair ValuePercent of Par(in millions)Senior notes:2021 Convertible Notes$180.890.4%2022 Senior Notes520.6104.1%2024 Senior Notes406.0101.5%
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|Senior notes:                                                                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|                                                                                                                                         | 2021 Convertible Notes                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|                                                                                                                                         | 2022 Senior Notes                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|                                                                                                                                         | 2024 Senior Notes                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

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