Income Tax Provision	3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Tax Provision	Income Tax Provision For the three months ended March 31, 2018, we had an effective income tax rate, as adjusted, of 4.1% compared to 10.6% for the three months ended March 31, 2017. The effective income tax rates for the periods presented were different from the statutory United States federal income tax rate of 21% and 35%, respectively, principally due to excess tax benefits related to stock option exercises. The excess tax benefits related to stock option exercises realized was $9.1 million for the three months ended March 31, 2018, compared to $10.1 million for the three months ended March 31, 2017. Excluding the excess tax benefits, the effective rate was 27.2% for the three months ended March 31, 2018, compared to 38.2% (as adjusted) for the three months ended March 31, 2017. Other differences from our federal statutory income tax rate included state income taxes, non-deductible business expenses, the tax benefit of research tax credits, and in 2017, the tax benefit of the domestic production activities deduction. The decrease in effective tax rate for the three months ended March 31, 2018 as compared to the same period 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 and the increase in 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 March 31, 2018, we have not reported any adjustments to the provisional amounts for the income tax effects of the Tax Act recorded in 2017. However, based on a continued analysis of the estimates and further guidance on the application of the law, it is anticipated that additional revisions may occur throughout the allowable measurement period. Overall, the changes due to the Tax Act will favorably affect income tax expense and future U.S. earnings. We made tax payments of $218,000 and $161,000 in the three months ended March 31, 2018 and 2017, respectively.

Income Tax Provision

For the three months ended March 31, 2018, we had an effective income tax rate, as adjusted, of 4.1% compared to 10.6% for the three months ended March 31, 2017. The effective income tax rates for the periods presented were different from the statutory United States federal income tax rate of 21% and 35%, respectively, principally due to excess tax benefits related to stock option exercises. The excess tax benefits related to stock option exercises realized was $9.1 million for the three months ended March 31, 2018, compared to $10.1 million for the three months ended March 31, 2017. Excluding the excess tax benefits, the effective rate was 27.2% for the three months ended March 31, 2018, compared to 38.2% (as adjusted) for the three months ended March 31, 2017. Other differences from our federal statutory income tax rate included state income taxes, non-deductible business expenses, the tax benefit of research tax credits, and in 2017, the tax benefit of the domestic production activities deduction.

The decrease in effective tax rate for the three months ended March 31, 2018 as compared to the same period 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 and the increase in 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 March 31, 2018, we have not reported any adjustments to the provisional amounts for the income tax effects of the Tax Act recorded in 2017. However, based on a continued analysis of the estimates and further guidance on the application of the law, it is anticipated that additional revisions may occur throughout the allowable measurement period. Overall, the changes due to the Tax Act will favorably affect income tax expense and future U.S. earnings.

We made tax payments of $218,000 and $161,000 in the three months ended March 31, 2018 and 2017, respectively.