Income Tax Provision	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Tax Provision	Income Tax Provision We had an effective income tax rate of negative 0.8% and 0.1% for the three and nine months ended September 30, 2018, respectively, compared to 12.4% and 12.5% (as adjusted) for the three and nine months ended September 30, 2017, respectively. The effective income tax rates for the periods presented were different from the statutory United States federal income tax rate of 21% in 2018 and 35% in 2017 principally due to excess tax benefits related to stock option exercises. The excess tax benefit related to stock option exercises realized was $9.3 million and $30.0 million for the three and nine months ended September 30, 2018, respectively, compared to $9.0 million and $27.6 million for the three and nine months ended September 30, 2017, respectively. Excluding the excess tax benefits, the effective rate was 23.3% and 25.8% for the three and nine months ended September 30, 2018, respectively, compared to 32.8% and 35.9% (as adjusted) for the three and nine months ended September 30, 2017, respectively. Other differences from the federal statutory income tax rate include state income taxes, non-deductible business expenses, the tax benefit of research tax credits, provisional amounts associated with the Tax Act recorded in 2018, and in 2017, the tax benefit of the domestic production activities deduction. The decrease in effective tax rate for the three and nine months ended September 30, 2018, as compared to the same periods in 2017 was due primarily to the reduction of the U.S. corporate tax rate from 35% to 21% as a result of the Tax Act, the increase in excess tax benefit related to stock option exercises and the research tax credit benefit, offset by the elimination of the domestic production activities deduction and the increased limitations on the deduction for executive compensation. In the fourth quarter of 2017, we recorded a $26.0 million (as adjusted under Topic 606) tax benefit due to the remeasurement of deferred tax assets and liabilities at a lower tax rate. As of September 30, 2018, we increased the provisional amounts for the income tax effects of the Tax Act recorded in 2017 to $27.7 million to reflect actual amounts reported in income tax returns filings. We will continue to analyze the estimates and further guidance on the application of the law, and additional revisions may occur throughout the allowable measurement period which will not exceed one year from the enactment of the Tax Act. Overall, the changes due to the Tax Act will favorably affect income tax expense and future U.S. earnings. We made tax payments of $6.8 million and $29.0 million in the nine months ended September 30, 2018, and 2017, respectively.

Income Tax Provision

We had an effective income tax rate of negative 0.8% and 0.1% for the three and nine months ended September 30, 2018, respectively, compared to 12.4% and 12.5% (as adjusted) for the three and nine months ended September 30, 2017, respectively. The effective income tax rates for the periods presented were different from the statutory United States federal income tax rate of 21% in 2018 and 35% in 2017 principally due to excess tax benefits related to stock option exercises. The excess tax benefit related to stock option exercises realized was $9.3 million and $30.0 million for the three and nine months ended September 30, 2018, respectively, compared to $9.0 million and $27.6 million for the three and nine months ended September 30, 2017, respectively. Excluding the excess tax benefits, the effective rate was 23.3% and 25.8% for the three and nine months ended September 30, 2018, respectively, compared to 32.8% and 35.9% (as adjusted) for the three and nine months ended September 30, 2017, respectively. Other differences from the federal statutory income tax rate include state income taxes, non-deductible business expenses, the tax benefit of research tax credits, provisional amounts associated with the Tax Act recorded in 2018, and in 2017, the tax benefit of the domestic production activities deduction.

The decrease in effective tax rate for the three and nine months ended September 30, 2018, as compared to the same periods in 2017 was due primarily to the reduction of the U.S. corporate tax rate from 35% to 21% as a result of the Tax Act, the increase in excess tax benefit related to stock option exercises and the research tax credit benefit, offset by the elimination of the domestic production activities deduction and the increased limitations on the deduction for executive compensation. In the fourth quarter of 2017, we recorded a $26.0 million (as adjusted under Topic 606) tax benefit due to the remeasurement of deferred tax assets and liabilities at a lower tax rate. As of September 30, 2018, we increased the provisional amounts for the income tax effects of the Tax Act recorded in 2017 to $27.7 million to reflect actual amounts reported in income tax returns filings. We will continue to analyze the estimates and further guidance on the application of the law, and additional revisions may occur throughout the allowable measurement period which will not exceed one year from the enactment of the Tax Act. Overall, the changes due to the Tax Act will favorably affect income tax expense and future U.S. earnings.

We made tax payments of $6.8 million and $29.0 million in the nine months ended September 30, 2018, and 2017, respectively.