As a limited partnership, we generally are not subject to federal, state or local income taxes. The tax on the net income of the Partnership is generally borne by the individual partners. We have Canadian activity that is taxable in Canada. In addition, we own three entities which have elected to be taxed as corporations for U.S. federal income tax purposes. The amounts recognized as income tax expense, income taxes payable, and deferred tax liabilities in the Consolidated Financial Statements represent the Canadian and U.S. taxes referred to above, as well as partnership-level taxes levied by various states (primarily Texas).

Significant components of income tax expense (benefit) are as follows for the years ended December 31:

The increase in total income tax expense from 2015 to 2016 is primarily attributable to improved operating results of Tulsa Inspection Resources – PUC, LLC, an entity that has elected to be taxable as a corporation for federal and state income tax purposes. Revenues and net taxable income of this entity have increased from the year ended December 31, 2015 to the year ended December 31, 2016. The decrease in total income tax expense from 2016 to 2017 is primarily attributable to deferred tax effects of intangible asset impairments from our Canadian subsidiary.

Noncurrent deferred tax liabilities of $0.4 million at December 31, 2016 are primarily attributable to the recorded unamortized portion of book intangible assets in our Canadian subsidiary. These intangible assets were impaired in 2017 with the resulting deferred tax benefit recorded in 2017.

The following table reconciles the differences between the U.S. federal statutory rate of 35% to the Partnership’s income tax expense on the Consolidated Statements of Operations for the years ended December 31:

Tax years that remain subject to examination by various taxing authorities for each of our consolidated entities include the years 2015 through 2017. It is the Partnership’s policy to recognize tax-related interest and penalties as a component of income tax expense in the year incurred. Tax-related interest and penalties were insignificant in the years ended December 31, 2017, 2016 and 2015.

As of December 31, 2017 or December 31, 2016, the Partnership had no significant unrecognized tax benefits. During the next twelve months, we do not expect that the ultimate resolution of any uncertain tax positions will result in a significant increase or decrease of an unrecognized tax benefit.