Income Taxes	3 Months Ended
Mar. 31, 2019
Income Taxes [Abstract]
Income Taxes	6. Income Taxes Our effective tax rate may vary from the U.S. federal statutory tax rate due to the change in the mix of earnings in tax jurisdictions with different statutory rates, benefits related to tax credits, and the tax impact of non-deductible expenses and other permanent differences between income before income taxes and taxable income. Our effective income tax rate of 25.5 percent for the three months ended March 31, 2019 differed from the U.S. federal statutory rate of 21 percent primarily due to state taxes, partially offset by the tax benefit related to estimated research & development and orphan drug credits and the excess tax benefit related to share-based compensation. We recorded income tax expense of $31 million for the three months ended March 31, 2019, compared to $15,000 for the same period in 2018. The increase in our income tax expense was primarily due to our expectation that we will generate U.S. federal and state taxable income in 2019. Our 2019 income tax expense has two components. The first component relates to federal income taxes. We expect to utilize our deferred tax assets to offset our U.S. federal taxable income. We are recording non-cash income tax expense as we utilize our federal deferred tax assets. The other component of our income tax expense relates to the estimated cash taxes we will pay for our state income taxes. Although we are recording the expense for our state income taxes in 2019, we will not have to make the majority of the payment for this liability until the first quarter of 2020.

6. Income Taxes

Our effective tax rate may vary from the U.S. federal statutory tax rate due to the change in the mix of earnings in tax jurisdictions with different statutory rates, benefits related to tax credits, and the tax impact of non-deductible expenses and other permanent differences between income before income taxes and taxable income. Our effective income tax rate of 25.5 percent for the three months ended March 31, 2019 differed from the U.S. federal statutory rate of 21 percent primarily due to state taxes, partially offset by the tax benefit related to estimated research & development and orphan drug credits and the excess tax benefit related to share-based compensation.

We recorded income tax expense of $31 million for the three months ended March 31, 2019, compared to $15,000 for the same period in 2018. The increase in our income tax expense was primarily due to our expectation that we will generate U.S. federal and state taxable income in 2019. Our 2019 income tax expense has two components. The first component relates to federal income taxes. We expect to utilize our deferred tax assets to offset our U.S. federal taxable income. We are recording non-cash income tax expense as we utilize our federal deferred tax assets. The other component of our income tax expense relates to the estimated cash taxes we will pay for our state income taxes. Although we are recording the expense for our state income taxes in 2019, we will not have to make the majority of the payment for this liability until the first quarter of 2020.