Income Taxes	3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company’s effective tax rate for the three months ended March 31, 2018 was 18.2% as compared to an effective tax rate of 40.2% for the same period in 2017. The effective tax rate for the three months ended March 31, 2018 was lower than the statutory federal rate of 21% primarily due to (i) the tax impact of a decrease in the Company’s deferred state tax rate as a result of the Permian Basin Acquisition and (ii) the portion of OMP’s earnings attributable to the non-controlling public limited partners, which are not taxable to the Company. These decreases are partially offset by (i) state income taxes, (ii) an increase in the valuation allowance recorded against the Company’s Montana net operating loss carryforwards and (iii) a permanent difference related to equity-based compensation shortfalls. The effective tax rate for the three months ended March 31, 2017 was higher than the statutory federal rate of 35% primarily due to state taxes and a permanent difference related to nondeductible executive compensation. These increases were partially offset by a permanent difference related to equity-based compensation windfalls. Valuation allowance. The Company had valuation allowances of $3.3 million and $1.2 million as of March 31, 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 three months ended March 31, 2018, the valuation allowance was increased by $2.2 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. 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. There have been no material changes to the provisional estimate as disclosed in the Company’s 2017 Form 10-K. The Company will continue to analyze the impact of the new law and additional impacts will be recorded as they are identified during the measurement period provided for in SAB 118.

Income Taxes

The Company’s effective tax rate for the three months ended March 31, 2018 was 18.2% as compared to an effective tax rate of 40.2% for the same period in 2017. The effective tax rate for the three months ended March 31, 2018 was lower than the statutory federal rate of 21% primarily due to (i) the tax impact of a decrease in the Company’s deferred state tax rate as a result of the Permian Basin Acquisition and (ii) the portion of OMP’s earnings attributable to the non-controlling public limited partners, which are not taxable to the Company. These decreases are partially offset by (i) state income taxes, (ii) an increase in the valuation allowance recorded against the Company’s Montana net operating loss carryforwards and (iii) a permanent difference related to equity-based compensation shortfalls.

The effective tax rate for the three months ended March 31, 2017 was higher than the statutory federal rate of 35% primarily due to state taxes and a permanent difference related to nondeductible executive compensation. These increases were partially offset by a permanent difference related to equity-based compensation windfalls.

Valuation allowance. The Company had valuation allowances of $3.3 million and $1.2 million as of March 31, 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 three months ended March 31, 2018, the valuation allowance was increased by $2.2 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.

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. There have been no material changes to the provisional estimate as disclosed in the Company’s 2017 Form 10-K. The Company will continue to analyze the impact of the new law and additional impacts will be recorded as they are identified during the measurement period provided for in SAB 118.