Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2020
Income Taxes [Abstract]
Income Taxes	5. Income Taxes The Coronavirus Aid, Relief, and Economic Security, or CARES, Act was enacted in March 2020. We considered our ability to estimate annual effective tax rates based on our pre-tax income projections, the income tax effects of the CARES Act, the realizability of our net deferred tax assets and the appropriateness of our valuation allowances. Under the Tax Cut and Jobs Act of 2017, the utilization of federal net operating losses was limited to 80 percent of taxable income. The CARES Act temporarily removed this limitation and provides for the utilization of net operating loss carryforwards to offset 100 percent of taxable income. The 80 percent limitation enacted by the Tax Act is reinstated for tax years beginning in 2021. In 2019, we recorded income tax expense related to Akcea due to the 80 percent limitation on the utilization of net operating losses in effect at the time. As a result of the temporary change in tax law provided by the CARES Act, we recorded a $1.7 million tax benefit in the first quarter of 2020 as we will now utilize federal net operating loss carryforwards to offset 100 percent of Akcea’s taxable income for 2019. We recorded the tax benefit as a discrete item in the first quarter of 2020 because that was when the CARES Act was enacted. We recorded an income tax benefit of $0.4 million and $3.7 million for the three and six months ended June 30, 2020, respectively, compared to an income tax benefit of $6.9 million and income tax expense of $24.1 million for the same periods in 2019. We recorded an income tax benefit for the first half of 2020 primarily due to Ionis’ pre-tax loss for the period and the $1.7 million tax benefit related to Akcea. We did not record a tax benefit as a result of Akcea’s pre-tax loss in the first half of 2020 because Akcea maintains a full valuation allowance against its deferred tax assets. Our effective tax rate may vary from the U.S. federal statutory rate due to the change in the mix of earnings in tax jurisdictions with different statutory rates, benefits related to tax credits, the tax impact of non-deductible expenses and other permanent differences between income before taxes and taxable income, and changes to tax laws or rates. Our effective income tax rate of 3.5 percent for the six months ended June 30, 2020 differed from the U.S. federal statutory rate of 21 percent primarily due to Ionis’ pre-tax loss for the period and the $1.7 million tax benefit related to Akcea.

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.