Income Tax	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Tax	Income TaxOur effective tax rate may vary from the U.S. federal statutory tax rate due to the change in the mix of earnings in various state jurisdictions with different statutory rates, benefits related to tax credits, and the tax impact of non-deductible expenses, stock award activities and other permanent differences between income before income taxes and taxable income. The effective tax rate for the three and nine months ended September 30, 2019 was 23.2% and 20.9%, respectively. The variance from the U.S. federal statutory tax rate of 21% for the three months ended September 30, 2019 was primarily attributable to state taxes and a lower R&D tax credit. The variance from the U.S. federal statutory tax rate of 21% for the nine months ended September 30, 2019 was primarily attributable to tax deductions related to stock award activities and the release of a valuation allowance relating to our R&D tax credits which were recorded as discrete items. The effective tax rate for the three and nine months ended September 30, 2018 was 15.0% and 19.3%, respectively. The variance from the U.S. federal statutory tax rate of 21% for the three and nine months ended September 30, 2018 was primarily attributable to tax deductions related to stock award activities which were recorded as discrete items during the periods as well as the release of a valuation allowance relating to our investment in Viking during the first quarter of 2018.