Income Taxes	3 Months Ended
Dec. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES Income tax expense includes a provision for federal, state and foreign taxes based on the annual estimated effective tax rate applicable to us and our subsidiaries, adjusted for items which are considered discrete to the period. On December 22, 2017, the Tax Cuts and Jobs Act (the “Tax Act”) was enacted. The Tax Act contains significant changes to U.S. tax law, including lowering the U.S. corporate income tax rate to 21.0% and implementing a territorial tax system. Since we have a September year-end, the lower U.S. corporate income tax rate will be phased in. Our U.S. federal blended tax rate will be approximately 24.5% for our fiscal year ending September 29, 2018 and 21.0% for subsequent fiscal years. The reduction of the U.S. corporate income tax rate will cause us to adjust our U.S. deferred tax assets and liabilities to the lower U.S. federal tax rate of 21.0%. There are also certain transitional impacts of the Tax Act. As part of the transition to the new territorial tax system, the Tax Act imposes a one-time deemed repatriation tax on our foreign subsidiaries’ historical earnings. These transitional impacts resulted in a provisional net charge of $41.7 million for the quarter ended December 30, 2017. This is comprised of an estimated deemed repatriation tax charge of $48.7 million less a previously recorded deferred tax liability of $20.3 million for anticipated repatriation of our investment in a foreign subsidiary, plus an estimated deferred tax remeasurement charge of $13.3 million. The Tax Act changes are broad and complex. The final calculation of impacts of the Tax Act may materially differ from the above provisional estimates. Among other things, this may be due to changes in interpretations of the Tax Act, any legislative action to address questions that arise because of the Tax Act, any changes in accounting standards for income taxes or related interpretations in response to the Tax Act, or any updates or changes to estimates we have utilized to calculate the transitional impacts. The Securities Exchange Commission has issued guidance under Staff Accounting Bulletin No. 118 directing taxpayers to record impacts of the Tax Act as “provisional” when it does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete the accounting under ASC 740. The guidance allows for a measurement period of up to one year after the enactment date of the Tax Act to finalize the recording of the related tax impacts. We currently anticipate finalizing and recording any resulting adjustments by the end of our current fiscal year ending September 29, 2018. The Tax Act also includes provisions for Global Intangible Low-Taxed Income (“GILTI”) wherein taxes on foreign income are imposed in excess of a deemed return on tangible assets of foreign corporations. In general, this income will effectively be taxed at a 10.5% tax rate reduced by any available current year foreign tax credits. This provision is effective for taxable years beginning after December 31, 2017. Because of the complexity of the new GILTI tax rules, we continue to evaluate this provision of the Tax Act including the associated forecast of GILTI and the application of ASC 740, Income Taxes. Under U.S. GAAP, we are allowed to make an accounting policy choice of either (1) treating taxes due on future U.S. inclusions in taxable income related to GILTI as a current-period expense when incurred (the “period cost method”) or (2) factoring such amounts into our measurement of our deferred taxes (the “deferred method”). Our selection of an accounting policy with respect to the new GILTI tax rules will depend, in part, on analyzing our global income to determine whether we expect to have future U.S. inclusions in taxable income related to GILTI and, if so, what the impact is expected to be. Whether we expect to have future U.S. inclusions in taxable income related to GILTI depends on not only our current structure and estimated future results of global operations, but also our intent and ability to modify our structure. We are currently in the process of analyzing our structure and, as a result, are not yet able to reasonably estimate the effect of this provision of the Tax Act. Therefore, we have not made any adjustments related to potential GILTI tax in our financial statements and have not made a policy decision regarding whether to record deferred tax on GILTI. Our effective tax rate on income from continuing operations before income taxes for the three months ended December 30, 2017 was 58.4%. Our effective tax rate for the three months ended December 30, 2017 was higher than the effective U.S. federal blended tax rate of 24.5% primarily due to the Tax Act one-time mandatory deemed repatriation transition tax, the impact of income subject to foreign tax rates that are higher than the U.S. tax rates, the remeasurement of deferred tax assets and liabilities based on the newly enacted U.S. federal tax rate of 21.0%, stock-based compensation not deductible for tax purposes and limitations on the deductibility of compensation under IRC Section 162(m). These amounts are partially offset by the excess tax benefits from stock award exercises and restricted stock unit vesting, the benefit of foreign tax credits, the benefit of federal research and development tax credits, the benefit of a domestic manufacturing deduction under IRC Section 199 and the Singapore tax exemption. The effective tax rate on income from continuing operations before income taxes for the first quarter of fiscal 2017 of 35.2% was higher than the U.S. federal rate of 35.0% primarily due to Rofin transaction costs not deductible for tax purposes, tax costs of Rofin restructuring, ASC 740-10 (formerly FIN48) tax liabilities for transfer pricing, stock-based compensation not deductible for tax purposes and limitations on the deductibility of compensation under IRC Section 162(m). These amounts are partially offset by differences related to the benefit of income subject to foreign tax rates that are lower than U.S. tax rates including the Singapore tax exemption and the benefit of federal research and development tax credits. Determining the consolidated provision for income taxes, income tax liabilities and deferred tax assets and liabilities involves judgment. We calculate and provide for income taxes in each of the tax jurisdictions in which we operate, which involves estimating current tax exposures as well as making judgments regarding the recoverability of deferred tax assets in each jurisdiction. The estimates used could differ from actual results, which may have a significant impact on operating results in future periods. We adopted ASU No. 2016-09 in the first quarter of fiscal 2018. As a result of adopting the new standard, excess tax benefits from equity-based compensation are now reflected in the condensed consolidated statements of operations as a component of the provision for income taxes. The adoption of ASU No. 2016-09 resulted in a decrease in our provision for income taxes of $12.5 million for the three months ended December 30, 2017 due to the recognition of excess tax benefits for options exercised and the vesting of equity awards. We are subject to taxation and file income tax returns in the U.S. federal jurisdiction and in many state and foreign jurisdictions. For U.S. federal income tax purposes, all years prior to fiscal 2011 are closed. In September 2017, the Internal Revenue Service (IRS) completed its audit of Coherent, Inc.’s fiscal 2013 tax return with no adjustment. The extension of the statutes of limitations for its fiscal 2011 and 2012 tax returns will be closed on June 30, 2018. In our major foreign jurisdictions and our major state jurisdictions, the years prior to fiscal 2011 and 2013, respectively, are closed to examination. Earlier years in our various jurisdictions may remain open for adjustment to the extent that we have tax attribute carryforwards from those years. In July 2015 and March 2016, Coherent Kaiserslautern GmbH (formerly Lumera Laser GmbH) received tax audit notices for the fiscal years 2010 to 2014. The audit began in August 2015. We acquired the shares of Lumera Laser GmbH in December 2012 and, pursuant to the terms of the acquisition agreement, we should not have responsibility for any assessments related to the pre-acquisition period. In July, 2016, Coherent Holding GmbH and Coherent Deutschland GmbH each received a tax audit notice for the fiscal years 2011 to 2014. The audit began in August 2016. In November 2016, Coherent GmbH, Coherent LaserSystems GmbH & Co. KG and Coherent Germany GmbH received audit notices for the period that they were in existence during the fiscal years 2011 through 2014. The audit work began in January 2017. In the fourth quarter of fiscal 2017, all German tax audits were extended to fiscal 2015 and are currently in progress. We regularly engage in discussions and negotiations with tax authorities regarding tax matters in various jurisdictions and management believes that it has adequately provided reserves for any adjustments that may result from tax examinations.

INCOME TAXES

Income tax expense includes a provision for federal, state and foreign taxes based on the annual estimated effective tax rate applicable to us and our subsidiaries, adjusted for items which are considered discrete to the period.

On December 22, 2017, the Tax Cuts and Jobs Act (the “Tax Act”) was enacted. The Tax Act contains significant changes to U.S. tax law, including lowering the U.S. corporate income tax rate to 21.0% and implementing a territorial tax system. Since we have a September year-end, the lower U.S. corporate income tax rate will be phased in. Our U.S. federal blended tax rate will be approximately 24.5% for our fiscal year ending September 29, 2018 and 21.0% for subsequent fiscal years.

The reduction of the U.S. corporate income tax rate will cause us to adjust our U.S. deferred tax assets and liabilities to the lower U.S. federal tax rate of 21.0%. There are also certain transitional impacts of the Tax Act. As part of the transition to the new territorial tax system, the Tax Act imposes a one-time deemed repatriation tax on our foreign subsidiaries’ historical earnings. These transitional impacts resulted in a provisional net charge of $41.7 million for the quarter ended December 30, 2017. This is comprised of an estimated deemed repatriation tax charge of $48.7 million less a previously recorded deferred tax liability of $20.3 million for anticipated repatriation of our investment in a foreign subsidiary, plus an estimated deferred tax remeasurement charge of $13.3 million.

The Tax Act changes are broad and complex. The final calculation of impacts of the Tax Act may materially differ from the above provisional estimates. Among other things, this may be due to changes in interpretations of the Tax Act, any legislative action to address questions that arise because of the Tax Act, any changes in accounting standards for income taxes or related interpretations in response to the Tax Act, or any updates or changes to estimates we have utilized to calculate the transitional impacts. The Securities Exchange Commission has issued guidance under Staff Accounting Bulletin No. 118 directing taxpayers to record impacts of the Tax Act as “provisional” when it does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete the accounting under ASC 740. The guidance allows for a measurement period of up to one year after the enactment date of the Tax Act to finalize the recording of the related tax impacts. We currently anticipate finalizing and recording any resulting adjustments by the end of our current fiscal year ending September 29, 2018.

The Tax Act also includes provisions for Global Intangible Low-Taxed Income (“GILTI”) wherein taxes on foreign income are imposed in excess of a deemed return on tangible assets of foreign corporations. In general, this income will effectively be taxed at a 10.5% tax rate reduced by any available current year foreign tax credits. This provision is effective for taxable years beginning after December 31, 2017. Because of the complexity of the new GILTI tax rules, we continue to evaluate this provision of the Tax Act including the associated forecast of GILTI and the application of ASC 740, Income Taxes. Under U.S. GAAP, we are allowed to make an accounting policy choice of either (1) treating taxes due on future U.S. inclusions in taxable income related to GILTI as a current-period expense when incurred (the “period cost method”) or (2) factoring such amounts into our measurement of our deferred taxes (the “deferred method”). Our selection of an accounting policy with respect to the new GILTI tax rules will depend, in part, on analyzing our global income to determine whether we expect to have future U.S. inclusions in taxable income related to GILTI and, if so, what the impact is expected to be. Whether we expect to have future U.S. inclusions in taxable income related to GILTI depends on not only our current structure and estimated future results of global operations, but also our intent and ability to modify our structure. We are currently in the process of analyzing our structure and, as a result, are not yet able to reasonably estimate the effect of this provision of the Tax Act. Therefore, we have not made any adjustments related to potential GILTI tax in our financial statements and have not made a policy decision regarding whether to record deferred tax on GILTI.

Our effective tax rate on income from continuing operations before income taxes for the three months ended December 30, 2017 was 58.4%. Our effective tax rate for the three months ended December 30, 2017 was higher than the effective U.S. federal blended tax rate of 24.5% primarily due to the Tax Act one-time mandatory deemed repatriation transition tax, the impact of income subject to foreign tax rates that are higher than the U.S. tax rates, the remeasurement of deferred tax assets and liabilities based on the newly enacted U.S. federal tax rate of 21.0%, stock-based compensation not deductible for tax purposes and limitations on the deductibility of compensation under IRC Section 162(m). These amounts are partially offset by the excess tax benefits from stock award exercises and restricted stock unit vesting, the benefit of foreign tax credits, the benefit of federal research and development tax credits, the benefit of a domestic manufacturing deduction under IRC Section 199 and the Singapore tax exemption.

The effective tax rate on income from continuing operations before income taxes for the first quarter of fiscal 2017 of 35.2% was higher than the U.S. federal rate of 35.0% primarily due to Rofin transaction costs not deductible for tax purposes, tax costs of Rofin restructuring, ASC 740-10 (formerly FIN48) tax liabilities for transfer pricing, stock-based compensation not deductible for tax purposes and limitations on the deductibility of compensation under IRC Section 162(m). These amounts are partially offset by differences related to the benefit of income subject to foreign tax rates that are lower than U.S. tax rates including the Singapore tax exemption and the benefit of federal research and development tax credits.

Determining the consolidated provision for income taxes, income tax liabilities and deferred tax assets and liabilities involves judgment. We calculate and provide for income taxes in each of the tax jurisdictions in which we operate, which involves estimating current tax exposures as well as making judgments regarding the recoverability of deferred tax assets in each jurisdiction. The estimates used could differ from actual results, which may have a significant impact on operating results in future periods.

We adopted ASU No. 2016-09 in the first quarter of fiscal 2018. As a result of adopting the new standard, excess tax benefits from equity-based compensation are now reflected in the condensed consolidated statements of operations as a component of the provision for income taxes. The adoption of ASU No. 2016-09 resulted in a decrease in our provision for income taxes of $12.5 million for the three months ended December 30, 2017 due to the recognition of excess tax benefits for options exercised and the vesting of equity awards.

We are subject to taxation and file income tax returns in the U.S. federal jurisdiction and in many state and foreign jurisdictions. For U.S. federal income tax purposes, all years prior to fiscal 2011 are closed. In September 2017, the Internal Revenue Service (IRS) completed its audit of Coherent, Inc.’s fiscal 2013 tax return with no adjustment. The extension of the statutes of limitations for its fiscal 2011 and 2012 tax returns will be closed on June 30, 2018. In our major foreign jurisdictions and our major state jurisdictions, the years prior to fiscal 2011 and 2013, respectively, are closed to examination. Earlier years in our various jurisdictions may remain open for adjustment to the extent that we have tax attribute carryforwards from those years.

In July 2015 and March 2016, Coherent Kaiserslautern GmbH (formerly Lumera Laser GmbH) received tax audit notices for the fiscal years 2010 to 2014. The audit began in August 2015. We acquired the shares of Lumera Laser GmbH in December 2012 and, pursuant to the terms of the acquisition agreement, we should not have responsibility for any assessments related to the pre-acquisition period. In July, 2016, Coherent Holding GmbH and Coherent Deutschland GmbH each received a tax audit notice for the fiscal years 2011 to 2014. The audit began in August 2016. In November 2016, Coherent GmbH, Coherent LaserSystems GmbH & Co. KG and Coherent Germany GmbH received audit notices for the period that they were in existence during the fiscal years 2011 through 2014. The audit work began in January 2017. In the fourth quarter of fiscal 2017, all German tax audits were extended to fiscal 2015 and are currently in progress.

We regularly engage in discussions and negotiations with tax authorities regarding tax matters in various jurisdictions and management believes that it has adequately provided reserves for any adjustments that may result from tax examinations.