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Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income taxes	Income taxes Our effective tax rate for the three and nine months ended September 30, 2018 was 24.7% and 23.0%, respectively, compared to 33.8% and 30.2%, respectively, for the same periods in 2017. For the three months ended September 30, 2018, our effective tax rate differs from the federal statutory rate primarily due to state income tax expense on current year earnings. For the nine months ended September 30, 2018, our effective tax rate differs from the federal statutory rate primarily due to state income tax expense on current year earnings, tax benefits due to Internal Revenue Service acceptance of an accounting method change applicable to the 2017 tax year, energy credits, and tax law changes. For the same periods in the prior year, our effective tax rate differed from the federal statutory rate primarily due to state income tax expense on current year earnings, the favorable resolution of certain state income tax matters, the domestic production activities deduction, and tax law changes. Our effective tax rates for the three and nine months ended September 30, 2018 are lower than the prior year periods primarily due to the federal statutory rate reduction from 35% in 2017 to 21% in 2018 due to the Tax Cuts and Jobs Act (the “Tax Act”), which was enacted on December 22, 2017. We have not fully completed our accounting for the income tax effects of the Tax Act. As discussed in the SEC Staff Accounting Bulletin No. 118, the accounting for the Tax Act should be completed within one year from the Tax Act enactment. During the three and nine months ended September 30, 2018, we have made no material adjustments to the provisional amounts recorded at December 31, 2017. Adjustments to the provisional amounts recorded at December 31, 2017 will be reflected upon the completion of our accounting for the Tax Act. At September 30, 2018 and December 31, 2017, we had deferred tax assets, net of deferred tax liabilities and valuation allowance, of $408.0 million and $645.3 million, respectively. The accounting for deferred taxes is based upon estimates of future results. Differences between estimated and actual results could result in changes in the valuation of deferred tax assets that could have a material impact on our consolidated results of operations or financial position. Changes in existing tax laws could also affect actual tax results and the realization of deferred tax assets over time. Unrecognized tax benefits represent the difference between tax positions taken or expected to be taken in a tax return and the benefits recognized for financial statement purposes. At September 30, 2018 and December 31, 2017, we had $20.7 million and $48.6 million, respectively, of gross unrecognized tax benefits and $5.4 million and $4.9 million, respectively, of related accrued interest and penalties. It is reasonably possible within the next twelve months that our gross unrecognized tax benefits may decrease by up to $12.0 million, excluding interest and penalties, primarily due to potential audit settlements.