6. Income Taxes

The U.S. federal statutory tax rate was 21% for the three months ended March 31, 2018 and 35% for the three months ended March 31, 2017. The Company’s effective tax rates for the three months ended March 31, 2018 was 28.9% of pretax income. The Company had a net tax benefit for the three months ended March 31, 2017 which represented 42.7% of pretax income.

The difference between the U.S. federal statutory tax rate and the Company’s effective tax rate for the three months ended March 31, 2018 was primarily due to state income taxes and the inability to utilize losses experienced in certain Canadian operations. The difference between the U.S. federal statutory tax rate and the Company’s effective tax rates for the three months ended March 31, 2017 was primarily due to inclusion of excess tax benefits relating to the vesting of stock awards and the exercise of stock options related to the Company’s stock compensation program, and state income taxes.

The Company had unrecognized tax benefits of approximately $0.8 million as of March 31, 2018 and December 31, 2017, which were included in other liabilities in the accompanying consolidated balance sheets.

The Company’s policy is to recognize interest and penalties related to income tax liabilities as a component of income tax expense in the consolidated statements of operations. The amount of interest and penalties charged to income tax expense because of the unrecognized tax benefits was not significant for the three months ended March 31, 2018 and 2017.

The Company is subject to taxation in various jurisdictions. The Company’s tax returns for 2015 and 2016 are subject to examination by U.S. federal authorities. The Company’s tax returns are subject to examination by various state authorities for the years 2013 through 2016.