Income Taxes	9 Months Ended
Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes In the third quarter and first nine months of 2018, our effective tax rate was (70)% on pre-tax income of $10.0 million, and (39)% on pre-tax income of $28.0 million, respectively, compared to 236% on pre-tax income of $0.7 million, and (63)% on a pre-tax loss of $6.9 million in the third quarter and first nine months of 2017, respectively. In the first nine months of 2018 and 2017, our effective tax rate was lower than the statutory federal income tax rates (21% and 35%, respectively) due to U.S. tax reform, as described below, and our corporate structure in which our foreign taxes are at a net effective tax rate lower than the U.S. rate. A significant amount of our foreign earnings is generated by our subsidiaries organized in Ireland. In 2018 and 2017, the foreign rate differential predominantly relates to these Irish earnings. Our foreign rate differential in 2018 and 2017 includes the continuing rate benefit from a business realignment completed on September 30, 2014 in which intellectual property was transferred between two wholly-owned foreign subsidiaries. For the first nine months of 2018 and 2017, this realignment resulted in tax benefits of approximately $9 million and $21 million, respectively. 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 by 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 will 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. We estimate no federal income taxes payable as a result of the deemed repatriation of undistributed earnings as we estimate that the tax will be offset by a combination of current year losses and existing attributes which had a full valuation allowance recorded against the related deferred tax assets. In the third quarter we reduced our estimate for state income taxes payable by $5.4 million to reflect additional guidance on the state implications of the Tax Act. In the first nine months of 2018, we recorded a reasonable estimate of state income taxes payable on the deemed repatriation of $1.7 million. We also recorded a deferred tax benefit of $14.1 million as a reasonable estimate of 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. We are continuing to assess the effects of the Tax Act on our indefinite reinvestment assertion and the realizability of our U.S. deferred tax assets. We are not able to make reasonable estimates at this time of the effects of certain provisions of the Tax Act that will apply to us beginning in our fiscal year ending September 30, 2019, including the Global Intangible Low Tax Income tax (the "GILTI" tax). The changes included in the Tax Act are broad and complex. The final transition impacts of the Tax Act may differ from the above estimates, possibly materially, due to, among other things, changes in interpretations of the Tax Act, any legislative action to address questions that arise because of the Tax Act, actions taken by U.S. state governments and taxing authorities in response to 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 transition impacts, including impacts from changes to current year earnings estimates and foreign currency exchange rates of foreign subsidiaries. The Securities Exchange Commission has issued rules that allow 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 30, 2018. 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 June 30, 2018 and September 30, 2017, we had unrecognized tax benefits of $9.7 million and $14.8 million, respectively. If all our unrecognized tax benefits as of June 30, 2018 were to become recognizable in the future, we would record a benefit to the income tax provision of $9.7 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 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. In the first quarter of 2018, as a result of the adoption of ASU 2016-09, we recognized previously unrecognized tax benefits of $37.0 million as increases in deferred tax assets for tax loss carryovers and tax credits, primarily in the U.S. A corresponding increase to the valuation allowance of $36.9 million was recorded to the extent that it was not more likely than not that these benefits would be realized.

Income Taxes

In the third quarter and first nine months of 2018, our effective tax rate was (70)% on pre-tax income of $10.0 million, and (39)% on pre-tax income of $28.0 million, respectively, compared to 236% on pre-tax income of $0.7 million, and (63)% on a pre-tax loss of $6.9 million in the third quarter and first nine months of 2017, respectively. In the first nine months of 2018 and 2017, our effective tax rate was lower than the statutory federal income tax rates (21% and 35%, respectively) due to U.S. tax reform, as described below, and our corporate structure in which our foreign taxes are at a net effective tax rate lower than the U.S. rate. A significant amount of our foreign earnings is generated by our subsidiaries organized in Ireland. In 2018 and 2017, the foreign rate differential predominantly relates to these Irish earnings. Our foreign rate differential in 2018 and 2017 includes the continuing rate benefit from a business realignment completed on September 30, 2014 in which intellectual property was transferred between two wholly-owned foreign subsidiaries. For the first nine months of 2018 and 2017, this realignment resulted in tax benefits of approximately $9 million and $21 million, respectively.

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 by 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 will 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.

We estimate no federal income taxes payable as a result of the deemed repatriation of undistributed earnings as we estimate that the tax will be offset by a combination of current year losses and existing attributes which had a full valuation allowance recorded against the related deferred tax assets. In the third quarter we reduced our estimate for state income taxes payable by $5.4 million to reflect additional guidance on the state implications of the Tax Act. In the first nine months of 2018, we recorded a reasonable estimate of state income taxes payable on the deemed repatriation of $1.7 million. We also recorded a deferred tax benefit of $14.1 million as a reasonable estimate of 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.

We are continuing to assess the effects of the Tax Act on our indefinite reinvestment assertion and the realizability of our U.S. deferred tax assets. We are not able to make reasonable estimates at this time of the effects of certain provisions of the Tax Act that will apply to us beginning in our fiscal year ending September 30, 2019, including the Global Intangible Low Tax Income tax (the "GILTI" tax).

The changes included in the Tax Act are broad and complex. The final transition impacts of the Tax Act may differ from the above estimates, possibly materially, due to, among other things, changes in interpretations of the Tax Act, any legislative action to address questions that arise because of the Tax Act, actions taken by U.S. state governments and taxing authorities in response to 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 transition impacts, including impacts from changes to current year earnings estimates and foreign currency exchange rates of foreign subsidiaries. The Securities Exchange Commission has issued rules that allow 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 30, 2018.

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 June 30, 2018 and September 30, 2017, we had unrecognized tax benefits of $9.7 million and $14.8 million, respectively. If all our unrecognized tax benefits as of June 30, 2018 were to become recognizable in the future, we would record a benefit to the income tax provision of $9.7 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 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.

In the first quarter of 2018, as a result of the adoption of ASU 2016-09, we recognized previously unrecognized tax benefits of $37.0 million as increases in deferred tax assets for tax loss carryovers and tax credits, primarily in the U.S. A corresponding increase to the valuation allowance of $36.9 million was recorded to the extent that it was not more likely than not that these benefits would be realized.