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Divestitures, Assets Held for Sale, and Acquisitions Acquisitions (Details) $ in Thousands | 3 Months Ended | 4 Months Ended | 6 Months Ended
--- | --- | --- | ---
Mar. 31, 2017 USD ($) a | Mar. 31, 2016 USD ($) | Apr. 26, 2017 a | Mar. 31, 2017 USD ($)
Business Acquisition [Line Items] |  |  |  |
Payments to Acquire Oil and Gas Property and Equipment | $ | $ 75,105 | $ 15,044 |  |
Subsequent Event [Member] | Other Howard County Trades [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Non-monetary Trade, Acreage Acquired | a |  |  | 3,020 |
Non-monetary Trade, Acreage Exchanged | a |  |  | 2,310 |
Rock Oil Acquisition 2016 [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Payments to Acquire Businesses, Gross | $ | $ 7,400 |  |  | $ 998,400
Other Midland Basin Acquisition 2017 [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Net Acres Acquired | a | 2,900 |  |  |
Payments to Acquire Oil and Gas Property and Equipment | $ | $ 59,600 |  |  |

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