Income Taxes	6 Months Ended
Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income taxes	Income taxes Our effective tax rate for the three and six months ended June 30, 2017 was 17.8% and 26.6%, respectively, compared to 37.9% and 34.7%, respectively, for the same periods in 2016. Our effective tax rate for the current period differed from the federal statutory tax rate primarily due to state income tax expense on current year earnings, the favorable resolution of certain state income tax matters, and tax law changes. For the same period in the prior year, our effective tax rate differed from the federal statutory tax rate primarily due to state income tax expense on current year earnings and the favorable resolution of certain state income tax matters. Our effective tax rates for the three and six months ended June 30, 2017 are lower than for the prior year periods primarily as the result of tax law changes and the favorable resolution of certain state income tax matters. At June 30, 2017 and December 31, 2016, we had deferred tax assets, net of deferred tax liabilities and valuation allowance, of $1.0 billion. 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 June 30, 2017 and December 31, 2016, we had $9.8 million and $21.5 million, respectively, of gross unrecognized tax benefits and $2.2 million and $12.2 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 $6.4 million, excluding interest and penalties, primarily due to potential audit settlements. As a result of the adoption of ASU No. 2016-09 (see Note 1), we recorded a cumulative-effect adjustment to increase retained earnings and deferred tax assets as of January 1, 2017 by $18.6 million for previously unrecognized excess tax benefits. We are currently under examination by the IRS as part of the Compliance Assurance Process ("CAP") and various state taxing jurisdictions, and anticipate finalizing certain examinations within the next twelve months. The final outcome of these examinations is not yet determinable. The statutes of limitation for our major tax jurisdictions generally remain open for examination for tax years 2010 through the current year. Net operating loss and credit carryforwards remain open to examination until the tax year of utilization closes.

Income taxes

Our effective tax rate for the three and six months ended June 30, 2017 was 17.8% and 26.6%, respectively, compared to 37.9% and 34.7%, respectively, for the same periods in 2016. Our effective tax rate for the current period differed from the federal statutory tax rate primarily due to state income tax expense on current year earnings, the favorable resolution of certain state income tax matters, and tax law changes. For the same period in the prior year, our effective tax rate differed from the federal statutory tax rate primarily due to state income tax expense on current year earnings and the favorable resolution of certain state income tax matters. Our effective tax rates for the three and six months ended June 30, 2017 are lower than for the prior year periods primarily as the result of tax law changes and the favorable resolution of certain state income tax matters.

At June 30, 2017 and December 31, 2016, we had deferred tax assets, net of deferred tax liabilities and valuation allowance, of $1.0 billion. 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 June 30, 2017 and December 31, 2016, we had $9.8 million and $21.5 million, respectively, of gross unrecognized tax benefits and $2.2 million and $12.2 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 $6.4 million, excluding interest and penalties, primarily due to potential audit settlements.

As a result of the adoption of ASU No. 2016-09 (see Note 1), we recorded a cumulative-effect adjustment to increase retained earnings and deferred tax assets as of January 1, 2017 by $18.6 million for previously unrecognized excess tax benefits.

We are currently under examination by the IRS as part of the Compliance Assurance Process ("CAP") and various state taxing jurisdictions, and anticipate finalizing certain examinations within the next twelve months. The final outcome of these examinations is not yet determinable. The statutes of limitation for our major tax jurisdictions generally remain open for examination for tax years 2010 through the current year. Net operating loss and credit carryforwards remain open to examination until the tax year of utilization closes.