13.	INCOME TAXES

The provision for income taxes is based on a current estimate of the annual effective income tax rate adjusted to reflect the impact of discrete items. Significant management judgment is required in estimating operating income in order to determine our effective income tax rate. Our effective income tax rate was 19.6% and 24.4% for the six months ended June 30, 2018 and 2017, respectively. The lower effective tax rate for the six months ended June 30, 2018 is primarily a result of the decrease in the federal corporate tax rate, partially offset by a reduction in the excess tax benefit recognized during the six months ended June 30, 2018 compared to the six months ended June 30, 2017.