12. Income Taxes

The following table provides a comparative summary of our income tax expense and effective tax rate for the three and six months ended June 30, 2017 and 2016, in thousands:

	Three Months ended June 30,						Six Months ended June 30,
	2017			2016			2017			2016
	(unaudited)						(unaudited)
Income tax expense	$	9,057		$	405		$	14,983		$	605
Effective tax rate	34.3		%	3.8		%	35.1		%	3.9		%

The income tax expense for the three and six months ended June 30, 2017 is attributed to the U.S. federal and state income tax. The increase in the income tax expense and the effective tax rate for the three and six months ended June 30, 2017 as compared to the same period in the prior year is primarily attributable to the release of the valuation allowance on the deferred tax assets during the third quarter of 2016.