Income Taxes

Our tax provision or benefit from income taxes for interim periods is determined using an estimate of our annual effective tax rate, adjusted for discrete items, if any, that are taken into account in the relevant period. Each quarter we update our estimate of the annual effective tax rate, and if our estimated tax rate changes, we make a cumulative adjustment. Our quarterly tax provision and our quarterly estimate of our annual effective tax rate, are subject to variation due to several factors, including variability in accurately predicting our pre-tax and taxable income and loss and the mix of jurisdictions to which they relate. Additionally, our effective tax rate can be more or less volatile based on the amount of pre-tax income or loss. For example, the impact of discrete items and non-deductible expenses on our effective tax rate is greater when our pre-tax income is lower.

The provision for income taxes differs from amounts computed by applying the U.S. federal tax rate of 21% for 2019 and 2018 to income before income taxes for the three months ended March 31, 2019 and 2018 due to the following (in thousands):



	2019					2018
Computed “expected” tax expense	$	48,141		21.0	%	$	48,157		21.0	%
Changes resulting from:
Foreign income tax differential	(4,692		)	(2.0	)%	2,016			0.9	%
Excess tax benefit related to stock-based compensation	(6,385		)	(2.8	)%	(4,624		)	(2.0	)%
State taxes net of federal benefits	3,551			1.5	%	3,373			1.5	%
Foreign-sourced nontaxable income	(56		)	—	%	(6,588		)	(2.9	)%
Foreign withholding	5,275			2.3	%	5,471			2.4	%
GILTI, net of foreign tax credits	2,433			1.1	%	4,921			2.1	%
Change in valuation allowance	3,289			1.4	%	—			—	%
Other	5,578			2.4	%	1,656			0.7	%
Provision for income taxes	$	57,134		24.9	%	$	54,382		23.7	%