Income Taxes	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company’s effective tax rate for the three and nine months ended September 30, 2018 was 27.2% and 23.4%, respectively, as compared to an effective tax rate of 31.5% and 41.2% for the three and nine months ended September 30, 2017, respectively. The effective tax rate for the three months ended September 30, 2018 was higher than the statutory federal rate of 21% primarily due to state income taxes and the impact of non-deductible executive compensation. The effective tax rate for the nine months ended September 30, 2018 was higher than the statutory rate primarily due to state income taxes and the impact of a change in the blended state rate at which the Company’s deferred taxes are recorded, partially offset by the impact of non-deductible executive compensation. The effective tax rate for the three months ended September 30, 2017 was lower than the statutory federal rate of 35% primarily due to the impact of non-deductible executive compensation and equity-based compensation shortfalls, partially offset by state income taxes. The effective tax rate for the nine months ended September 30, 2017 was higher than the statutory federal rate of 35% primarily due to state income taxes, the impact of non-deductible executive compensation and equity-based compensation windfalls, partially offset by the portion of OMP’s earnings attributable to the non-controlling public limited partners, which are not taxable to the Company. Valuation allowance. The Company had valuation allowances of $3.3 million and $1.2 million as of September 30, 2018 and December 31, 2017, respectively, because the Company has concluded it is more likely than not that it will be unable to utilize certain state net operating loss carryforwards and charitable contribution carryforwards. As of each reporting date, the Company’s management considers new evidence, both positive and negative, which could impact management’s view with regard to future realization of deferred tax assets. During the nine months ended September 30, 2018, the valuation allowance was increased by $2.1 million, primarily against the Company’s Montana net operating loss carryforwards, as a result of the Permian Basin Acquisition and the corresponding shift of projected future taxable income into other states. During the three months ended September 30, 2018, there was no material change to the valuation allowance. Tax Cuts and Jobs Act. On December 22, 2017, the U.S. government enacted the Tax Act, which made broad and complex changes to the U.S. tax code. Due to the complexities involved in the accounting for the enactment of the new law, the SEC issued SAB 118, which provides guidance on the accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date to complete the accounting under ASC 740, "Income Taxes." In accordance with SAB 118, the Company was able to make reasonable estimates on certain effects of the Tax Act in the financial statements as of December 31, 2017. During the three months ended September 30, 2018, the Company completed the accounting under the Tax Act. Based on additional guidance issued by the Internal Revenue Service regarding the grandfather provisions related to certain performance-based compensation awards outstanding as of November 2, 2017, the Company wrote off $1.9 million of deferred tax assets for which the Company will not receive a future tax deduction.

Income Taxes

The Company’s effective tax rate for the three and nine months ended September 30, 2018 was 27.2% and 23.4%, respectively, as compared to an effective tax rate of 31.5% and 41.2% for the three and nine months ended September 30, 2017, respectively. The effective tax rate for the three months ended September 30, 2018 was higher than the statutory federal rate of 21% primarily due to state income taxes and the impact of non-deductible executive compensation. The effective tax rate for the nine months ended September 30, 2018 was higher than the statutory rate primarily due to state income taxes and the impact of a change in the blended state rate at which the Company’s deferred taxes are recorded, partially offset by the impact of non-deductible executive compensation.

The effective tax rate for the three months ended September 30, 2017 was lower than the statutory federal rate of 35% primarily due to the impact of non-deductible executive compensation and equity-based compensation shortfalls, partially offset by state income taxes. The effective tax rate for the nine months ended September 30, 2017 was higher than the statutory federal rate of 35% primarily due to state income taxes, the impact of non-deductible executive compensation and equity-based compensation windfalls, partially offset by the portion of OMP’s earnings attributable to the non-controlling public limited partners, which are not taxable to the Company.

Valuation allowance. The Company had valuation allowances of $3.3 million and $1.2 million as of September 30, 2018 and December 31, 2017, respectively, because the Company has concluded it is more likely than not that it will be unable to utilize certain state net operating loss carryforwards and charitable contribution carryforwards. As of each reporting date, the Company’s management considers new evidence, both positive and negative, which could impact management’s view with regard to future realization of deferred tax assets. During the nine months ended September 30, 2018, the valuation allowance was increased by $2.1 million, primarily against the Company’s Montana net operating loss carryforwards, as a result of the Permian Basin Acquisition and the corresponding shift of projected future taxable income into other states. During the three months ended September 30, 2018, there was no material change to the valuation allowance.

Tax Cuts and Jobs Act. On December 22, 2017, the U.S. government enacted the Tax Act, which made broad and complex changes to the U.S. tax code. Due to the complexities involved in the accounting for the enactment of the new law, the SEC issued SAB 118, which provides guidance on the accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date to complete the accounting under ASC 740, "Income Taxes." In accordance with SAB 118, the Company was able to make reasonable estimates on certain effects of the Tax Act in the financial statements as of December 31, 2017. During the three months ended September 30, 2018, the Company completed the accounting under the Tax Act. Based on additional guidance issued by the Internal Revenue Service regarding the grandfather provisions related to certain performance-based compensation awards outstanding as of November 2, 2017, the Company wrote off $1.9 million of deferred tax assets for which the Company will not receive a future tax deduction.