Income Taxes	6 Months Ended
Mar. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes In the second quarter and first six months of 2019, our effective tax rate was (30)% on a pre-tax loss of $(33.4) million, and (73)% on a pre-tax loss of $(13.0) million, respectively, compared to 31% on pre-tax income of $11.5 million and (21)% on pre-tax income of $18.0 million in the second quarter and first six months of 2018, respectively. In the first six months of 2019 and 2018, our effective tax rate differed from the statutory federal income tax rate of 21% due to U.S. tax reform (as described below), our corporate structure in which our foreign taxes are at a net effective tax rate lower than the U.S. rate, the excess tax benefit related to stock-based compensation and, in the first six months of 2019, the reduction of a valuation allowance of $1.8 million as the result of the Frustum acquisition. Additionally, in the second quarter and first six months of 2019 our effective rate includes the indirect effects of the adoption of ASC 606. A significant amount of our foreign earnings is generated by our subsidiaries organized in Ireland. In 2019 and 2018, the foreign rate differential predominantly relates to these Irish earnings. On December 22, 2017, the United States enacted tax reform legislation through the Tax Cuts and Jobs Act, (the "Tax Act"), which significantly changed existing U.S. tax laws by a reduction of the corporate tax rate, the implementation of a new system of taxation for non-U.S. earnings, the imposition of a one-time tax on the deemed repatriation of undistributed earnings of non-U.S. subsidiaries, and the expansion of the limitations on the deductibility of executive compensation and interest expense. As we have a September 30 fiscal year-end, there is a blended U.S. statutory federal rate of approximately 24.5% for our fiscal year ending September 30, 2018 and 21% for subsequent fiscal years. The Tax Act also provides that net operating losses generated in years ending after December 31, 2017 may be carried forward indefinitely and can no longer be carried back, and that net operating losses generated in years beginning after December 31, 2017 can only reduce taxable income by up to 80% when utilized in a future period. The Tax Act includes a provision to tax global intangible low-tax income (GILTI) of foreign subsidiaries and a base erosion anti-abuse tax (BEAT) measure that taxes certain payments between a U.S. corporation and its foreign subsidiaries. The GILTI and BEAT provisions were effective for us beginning October 1, 2018. Our accounting policy is to treat tax on GILTI as a current period cost included in tax expense in the year incurred. In the first six months of 2018, we provided no federal income taxes payable as a result of the deemed repatriation of undistributed earnings as the tax was offset by a combination of current year losses and existing attributes which had a full valuation allowance recorded against the related deferred tax assets. We recorded state income taxes payable of $7.1 million on the deemed repatriation. This was subsequently reduced to $1.5 million to reflect additional guidance on the state implications of the Tax Act. We also recorded a deferred tax benefit of $14.1 million for the impact of the Tax Act on our net U.S. deferred income tax balances. This was primarily attributable to the reduction of the federal tax rate on the net deferred tax liability in the U.S., and the ability to realize net operating losses from the reversal of existing deferred tax assets which can now be carried forward indefinitely and can therefore be netted against deferred tax liabilities for indefinite lived intangible assets. The U.S. Securities and Exchange Commission issued rules that allow for a period of up to one year after the enactment date of the Tax Act to finalize the recording of the related tax impacts. We finalized recording the impacts of the Tax Act in the quarter ended December 29, 2018 and did not record any significant adjustments. In October 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory. The purpose of ASU 2016-16 is to simplify the income tax accounting of an intra-entity transfer of an asset other than inventory and to record its effect when the transfer occurs. We adopted this amendment beginning in the first quarter of 2019 using the modified retrospective method with a cumulative effect adjustment to accumulated deficit of $72.3 million, with a corresponding increase of $75.3 million to deferred tax assets, $6.0 million decrease to income tax assets and a $3.0 million decrease to income tax liabilities. The adjustment primarily relates to deductible amortization of intangible assets in Ireland. Post adoption, our effective tax rate no longer includes the benefit of this amortization. We have concluded, based on the weight of available evidence, that a full valuation allowance continues to be required against our U.S. net deferred tax assets as they are not more likely than not to be realized in the future. We will continue to reassess our valuation allowance requirements each financial reporting period. In the normal course of business, PTC and its subsidiaries are examined by various taxing authorities, including the Internal Revenue Service in the U.S. We regularly assess the likelihood of additional assessments by tax authorities and provide for these matters as appropriate. We are currently under audit by tax authorities in several jurisdictions. Audits by tax authorities typically involve examination of the deductibility of certain permanent items, limitations on net operating losses and tax credits. Although we believe our tax estimates are appropriate, the final determination of tax audits and any related litigation could result in material changes in our estimates. As of March 30, 2019 and September 30, 2018, we had unrecognized tax benefits of $10.3 million and $9.8 million, respectively. If all our unrecognized tax benefits as of March 30, 2019 were to become recognizable in the future, we would record a benefit to the income tax provision of $10.3 million, which would be partially offset by an increase in the U.S. valuation allowance of $3.8 million. Although we believe our tax estimates are appropriate, the final determination of tax audits and any related litigation could result in favorable or unfavorable changes in our estimates. We believe it is reasonably possible that within the next 12 months the amount of unrecognized tax benefits related to the resolution of multi-jurisdictional tax positions could be reduced by up to $2 million as audits close and statutes of limitations expire. In the fourth quarter of 2016, we received an assessment of approximately $12 million from the tax authorities in South Korea. The assessment relates to various tax issues, primarily foreign withholding taxes. We have appealed and intend to vigorously defend our positions. We believe that upon completion of a multi-level appeal process it is more likely than not that our positions will be sustained. Accordingly, we have not recorded a tax reserve for this matter. We paid this assessment in the first quarter of 2017 and have recorded the amount in other assets, pending resolution of the appeal process.

Income Taxes

In the second quarter and first six months of 2019, our effective tax rate was (30)% on a pre-tax loss of $(33.4) million, and (73)% on a pre-tax loss of $(13.0) million, respectively, compared to 31% on pre-tax income of $11.5 million and (21)% on pre-tax income of $18.0 million in the second quarter and first six months of 2018, respectively. In the first six months of 2019 and 2018, our effective tax rate differed from the statutory federal income tax rate of 21% due to U.S. tax reform (as described below), our corporate structure in which our foreign taxes are at a net effective tax rate lower than the U.S. rate, the excess tax benefit related to stock-based compensation and, in the first six months of 2019, the reduction of a valuation allowance of $1.8 million as the result of the Frustum acquisition. Additionally, in the second quarter and first six months of 2019 our effective rate includes the indirect effects of the adoption of ASC 606. A significant amount of our foreign earnings is generated by our subsidiaries organized in Ireland. In 2019 and 2018, the foreign rate differential predominantly relates to these Irish earnings.

On December 22, 2017, the United States enacted tax reform legislation through the Tax Cuts and Jobs Act, (the "Tax Act"), which significantly changed existing U.S. tax laws by a reduction of the corporate tax rate, the implementation of a new system of taxation for non-U.S. earnings, the imposition of a one-time tax on the deemed repatriation of undistributed earnings of non-U.S. subsidiaries, and the expansion of the limitations on the deductibility of executive compensation and interest expense. As we have a September 30 fiscal year-end, there is a blended U.S. statutory federal rate of approximately 24.5% for our fiscal year ending September 30, 2018 and 21% for subsequent fiscal years. The Tax Act also provides that net operating losses generated in years ending after December 31, 2017 may be carried forward indefinitely and can no longer be carried back, and that net operating losses generated in years beginning after December 31, 2017 can only reduce taxable income by up to 80% when utilized in a future period. The Tax Act includes a provision to tax global intangible low-tax income (GILTI) of foreign subsidiaries and a base erosion anti-abuse tax (BEAT) measure that taxes certain payments between a U.S. corporation and its foreign subsidiaries. The GILTI and BEAT provisions were effective for us beginning October 1, 2018. Our accounting policy is to treat tax on GILTI as a current period cost included in tax expense in the year incurred.

In the first six months of 2018, we provided no federal income taxes payable as a result of the deemed repatriation of undistributed earnings as the tax was offset by a combination of current year losses and existing attributes which had a full valuation allowance recorded against the related deferred tax assets. We recorded state income taxes payable of $7.1 million on the deemed repatriation. This was subsequently reduced to $1.5 million to reflect additional guidance on the state implications of the Tax Act. We also recorded a deferred tax benefit of $14.1 million for the impact of the Tax Act on our net U.S. deferred income tax balances. This was primarily attributable to the reduction of the federal tax rate on the net deferred tax liability in the U.S., and the ability to realize net operating losses from the reversal of existing deferred tax assets which can now be carried forward indefinitely and can therefore be netted against deferred tax liabilities for indefinite lived intangible assets.

The U.S. Securities and Exchange Commission issued rules that allow for a period of up to one year after the enactment date of the Tax Act to finalize the recording of the related tax impacts. We finalized recording the impacts of the Tax Act in the quarter ended December 29, 2018 and did not record any significant adjustments.

In October 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory. The purpose of ASU 2016-16 is to simplify the income tax accounting of an intra-entity transfer of an asset other than inventory and to record its effect when the transfer occurs. We adopted this amendment beginning in the first quarter of 2019 using the modified retrospective method with a cumulative effect adjustment to accumulated deficit of $72.3 million, with a corresponding increase of $75.3 million to deferred tax assets, $6.0 million decrease to income tax assets and a $3.0 million decrease to income tax liabilities. The adjustment primarily relates to deductible amortization of intangible assets in Ireland. Post adoption, our effective tax rate no longer includes the benefit of this amortization.

We have concluded, based on the weight of available evidence, that a full valuation allowance continues to be required against our U.S. net deferred tax assets as they are not more likely than not to be realized in the future. We will continue to reassess our valuation allowance requirements each financial reporting period.

In the normal course of business, PTC and its subsidiaries are examined by various taxing authorities, including the Internal Revenue Service in the U.S. We regularly assess the likelihood of additional assessments by tax authorities and provide for these matters as appropriate. We are currently under audit by tax authorities in several jurisdictions. Audits by tax authorities typically involve examination of the deductibility of certain permanent items, limitations on net operating losses and tax credits. Although we believe our tax estimates are appropriate, the final determination of tax audits and any related litigation could result in material changes in our estimates.

As of March 30, 2019 and September 30, 2018, we had unrecognized tax benefits of $10.3 million and $9.8 million, respectively. If all our unrecognized tax benefits as of March 30, 2019 were to become recognizable in the future, we would record a benefit to the income tax provision of $10.3 million, which would be partially offset by an increase in the U.S. valuation allowance of $3.8 million.

Although we believe our tax estimates are appropriate, the final determination of tax audits and any related litigation could result in favorable or unfavorable changes in our estimates. We believe it is reasonably possible that within the next 12 months the amount of unrecognized tax benefits related to the resolution of multi-jurisdictional tax positions could be reduced by up to $2 million as audits close and statutes of limitations expire.

In the fourth quarter of 2016, we received an assessment of approximately $12 million from the tax authorities in South Korea. The assessment relates to various tax issues, primarily foreign withholding taxes. We have appealed and intend to vigorously defend our positions. We believe that upon completion of a multi-level appeal process it is more likely than not that our positions will be sustained. Accordingly, we have not recorded a tax reserve for this matter. We paid this assessment in the first quarter of 2017 and have recorded the amount in other assets, pending resolution of the appeal process.