Income Taxes	3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The provision for income taxes is determined using an estimated annual effective tax rate, which is generally less than the U.S. federal statutory rate, primarily due to research and development (“R&D”) tax credits. Our effective tax rate may be subject to fluctuations during the year as new information is obtained, which may affect the assumptions used to estimate the annual effective tax rate, including factors such as expected utilization of R&D tax credits, valuation allowances against deferred tax assets, the recognition or derecognition of tax benefits related to uncertain tax positions, and changes in or the interpretation of tax laws in jurisdictions where we conduct business. Also, excess tax benefits and tax deficiencies related to our equity compensation recognized in the income statement could result in fluctuations in our effective tax rate period-over-period depending on the volatility of our stock price and how many awards vest in the period. We recognize deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of our assets and liabilities along with net operating loss and tax credit carryovers. We record a valuation allowance against our deferred tax assets to reduce the net carrying value to an amount that we believe is more likely than not to be realized. When we establish or reduce our valuation allowances against our deferred tax assets, the provision for income taxes will increase or decrease, respectively, in the period when that determination is made. On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“2017 Tax Act”) which, among a broad range of tax reform measures, reduced the U.S. corporate tax rate from 35.0% to 21.0% effective January 1, 2018. The reduction in the U.S. corporate tax rate required the federal portion of our deferred tax assets and liabilities at December 31, 2017 to be re-measured at the enacted tax rate expected to apply when the temporary differences are to be realized or settled using 21.0%. SEC Staff Accounting Bulletin No. 118, “Income Tax Accounting Implications of the Tax Cuts and Jobs Act” (“SAB 118”), allows us to record provisional amounts for the tax effects of the 2017 Tax Act when we do not have necessary information available, prepared, or analyzed in reasonable detail to finalize its accounting for the changes in the tax law during a measurement period not to extend beyond one year of the enactment date. During the three months ended March 31, 2018, we did not recognize any changes to the provisional amounts recorded in our 2017 Annual Report on Form 10-K in connection with the 2017 Tax Act. We expect to finalize our analysis within the measurement period in accordance with SAB 118 after completing our reviews of additional guidance issued by the Internal Revenue Service (“IRS”) and available tax methods and elections related to 2017 tax return positions. We recorded an income tax benefit of $0.2 million (effective tax benefit rate of 10.3%) for the three months ended March 31, 2018 compared to income tax expense of $0.4 million (effective tax rate of 15.6%) for the three months ended April 1, 2017. The decrease in the effective tax rate for the first quarter of 2018 compared to the first quarter of 2017 was primarily due to the reduction in the U.S. corporate tax rate from 35.0% to 21.0% and additional research and development tax credits recognized, partially offset by the repeal of the U.S. Domestic Production Activities Deduction. Our total amount of unrecognized tax benefits were $8.9 million and $5.3 million as of March 31, 2018 and December 31, 2017, respectively. If recognized, $3.4 million would affect the effective tax rate. The increase in unrecognized tax benefits is related to a change in our accounting treatment of advanced payments for tax purposes. We filed an Application for Change in Accounting Method (“Form 3115”) with the IRS to provide for different treatment of advanced payments for tax purposes compared to book. Upon filing the automatic method change, audit protection is provided and we expect to reclassify the unrecognized tax benefits to deferred tax liability in the subsequent quarter.

Income Taxes

The provision for income taxes is determined using an estimated annual effective tax rate, which is generally less than the U.S. federal statutory rate, primarily due to research and development (“R&D”) tax credits. Our effective tax rate may be subject to fluctuations during the year as new information is obtained, which may affect the assumptions used to estimate the annual effective tax rate, including factors such as expected utilization of R&D tax credits, valuation allowances against deferred tax assets, the recognition or derecognition of tax benefits related to uncertain tax positions, and changes in or the interpretation of tax laws in jurisdictions where we conduct business. Also, excess tax benefits and tax deficiencies related to our equity compensation recognized in the income statement could result in fluctuations in our effective tax rate period-over-period depending on the volatility of our stock price and how many awards vest in the period. We recognize deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of our assets and liabilities along with net operating loss and tax credit carryovers.

We record a valuation allowance against our deferred tax assets to reduce the net carrying value to an amount that we believe is more likely than not to be realized. When we establish or reduce our valuation allowances against our deferred tax assets, the provision for income taxes will increase or decrease, respectively, in the period when that determination is made.

On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“2017 Tax Act”) which, among a broad range of tax reform measures, reduced the U.S. corporate tax rate from 35.0% to 21.0% effective January 1, 2018. The reduction in the U.S. corporate tax rate required the federal portion of our deferred tax assets and liabilities at December 31, 2017 to be re-measured at the enacted tax rate expected to apply when the temporary differences are to be realized or settled using 21.0%.

SEC Staff Accounting Bulletin No. 118, “Income Tax Accounting Implications of the Tax Cuts and Jobs Act” (“SAB 118”), allows us to record provisional amounts for the tax effects of the 2017 Tax Act when we do not have necessary information available, prepared, or analyzed in reasonable detail to finalize its accounting for the changes in the tax law during a measurement period not to extend beyond one year of the enactment date. During the three months ended March 31, 2018, we did not recognize any changes to the provisional amounts recorded in our 2017 Annual Report on Form 10-K in connection with the 2017 Tax Act. We expect to finalize our analysis within the measurement period in accordance with SAB 118 after completing our reviews of additional guidance issued by the Internal Revenue Service (“IRS”) and available tax methods and elections related to 2017 tax return positions.

We recorded an income tax benefit of $0.2 million (effective tax benefit rate of 10.3%) for the three months ended March 31, 2018 compared to income tax expense of $0.4 million (effective tax rate of 15.6%) for the three months ended April 1, 2017. The decrease in the effective tax rate for the first quarter of 2018 compared to the first quarter of 2017 was primarily due to the reduction in the U.S. corporate tax rate from 35.0% to 21.0% and additional research and development tax credits recognized, partially offset by the repeal of the U.S. Domestic Production Activities Deduction.

Our total amount of unrecognized tax benefits were $8.9 million and $5.3 million as of March 31, 2018 and December 31, 2017, respectively. If recognized, $3.4 million would affect the effective tax rate. The increase in unrecognized tax benefits is related to a change in our accounting treatment of advanced payments for tax purposes. We filed an Application for Change in Accounting Method (“Form 3115”) with the IRS to provide for different treatment of advanced payments for tax purposes compared to book. Upon filing the automatic method change, audit protection is provided and we expect to reclassify the unrecognized tax benefits to deferred tax liability in the subsequent quarter.