Income Taxes	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	8. Income taxes We recorded an income tax expense of $6.0 million and $12.8 million for the three and nine months ended September 30, 2018, respectively, compared to a benefit of $0.1 million and an expense of $0.1 million for the three and nine months ended September 30, 2017, respectively. Income tax expense for the three and nine months ended September 30, 2018 was primarily due to the reduction of deferred tax assets of $5.0 million and $10.6 million, respectively, caused by our utilization of federal and state net operating losses and income tax expense of $1.0 million and $2.2 million, respectively, incurred in states where we have no net operating loss carryforwards to offset our tax obligations. Income tax expense for the three and nine months ended September 30, 2017 reflects income tax incurred in states where we have no net operating loss carryforwards to offset our tax obligations. Our effective tax rate differed from the federal statutory rate due to state income taxes and non-deductible stock-based compensation, which increased our tax expense, offset by research and development tax credits and the reduction in taxable income arising from the exercise of employee stock options during the reporting period. Each quarter, we assess our ability to use our deferred tax assets to offset our expected federal and state taxable income. In the fourth quarter of 2017, we determined that it was more likely than not that we would generate sufficient taxable income to utilize our federal and state deferred tax assets in every state except California. We therefore included in our balance sheet the net value of all our deferred tax assets except those applicable to California. All tax years from Corcept’s inception remain open to examination by the Internal Revenue Service, the California Franchise Tax Board and other state taxing authorities. The Tax Cuts and Jobs Act of 2017 (“Tax Act”), which became law on December 22, 2017, significantly changed federal income tax law. Among other things, effective January 1, 2018, the new law reduced the corporate income tax rate from 35 percent to 21 percent, repealed the corporate alternative minimum tax, limited some business deductions, modified the maximum deduction of future net operating losses generated with no carryback but an indefinite carryforward provision and extended the compensation deduction limit applicable to certain highly-compensated executives of publicly traded companies to cover additional executive roles. Accounting Standards Codification (ASC) 740, Income Taxes, requires companies to recognize the effect of tax law changes, such as those enacted by the Tax Act, in the period such changes take effect. We have adjusted our deferred taxes based on the reduction of the U.S. federal corporate tax rate from 35 percent to 21 percent and assessed the realizability of our deferred tax assets based on our current understanding of the new law. At September 30, 2018, the accounting for the impacts of the Tax Act was substantially complete and we do not expect material changes to the amounts recorded. We will continue to assess the Tax Act’s impact for the rest of 2018, including its interpretation by regulatory authorities, and will adjust our disclosures and financial presentation as necessary.

8. Income taxes

We recorded an income tax expense of $6.0 million and $12.8 million for the three and nine months ended September 30, 2018, respectively, compared to a benefit of $0.1 million and an expense of $0.1 million for the three and nine months ended September 30, 2017, respectively.

Income tax expense for the three and nine months ended September 30, 2018 was primarily due to the reduction of deferred tax assets of $5.0 million and $10.6 million, respectively, caused by our utilization of federal and state net operating losses and income tax expense of $1.0 million and $2.2 million, respectively, incurred in states where we have no net operating loss carryforwards to offset our tax obligations. Income tax expense for the three and nine months ended September 30, 2017 reflects income tax incurred in states where we have no net operating loss carryforwards to offset our tax obligations.

Our effective tax rate differed from the federal statutory rate due to state income taxes and non-deductible stock-based compensation, which increased our tax expense, offset by research and development tax credits and the reduction in taxable income arising from the exercise of employee stock options during the reporting period.

Each quarter, we assess our ability to use our deferred tax assets to offset our expected federal and state taxable income. In the fourth quarter of 2017, we determined that it was more likely than not that we would generate sufficient taxable income to utilize our federal and state deferred tax assets in every state except California. We therefore included in our balance sheet the net value of all our deferred tax assets except those applicable to California. All tax years from Corcept’s inception remain open to examination by the Internal Revenue Service, the California Franchise Tax Board and other state taxing authorities.

The Tax Cuts and Jobs Act of 2017 (“Tax Act”), which became law on December 22, 2017, significantly changed federal income tax law. Among other things, effective January 1, 2018, the new law reduced the corporate income tax rate from 35 percent to 21 percent, repealed the corporate alternative minimum tax, limited some business deductions, modified the maximum deduction of future net operating losses generated with no carryback but an indefinite carryforward provision and extended the compensation deduction limit applicable to certain highly-compensated executives of publicly traded companies to cover additional executive roles.

Accounting Standards Codification (ASC) 740, Income Taxes, requires companies to recognize the effect of tax law changes, such as those enacted by the Tax Act, in the period such changes take effect. We have adjusted our deferred taxes based on the reduction of the U.S. federal corporate tax rate from 35 percent to 21 percent and assessed the realizability of our deferred tax assets based on our current understanding of the new law. At September 30, 2018, the accounting for the impacts of the Tax Act was substantially complete and we do not expect material changes to the amounts recorded. We will continue to assess the Tax Act’s impact for the rest of 2018, including its interpretation by regulatory authorities, and will adjust our disclosures and financial presentation as necessary.