6. INCOME TAXES

The components of income tax expense (benefit) are as follows:



	Year Ended December 31,
thousands	2017		2016		2015
Current income tax expense (benefit)
Federal income tax expense (benefit)	$	—	$	4,477	$	32,422
State income tax expense (benefit)	2,408		1,340		1,764
Total current income tax expense (benefit)	2,408		5,817		34,186
Deferred income tax expense (benefit)
Federal income tax expense (benefit)	—		1,622		10,251
State income tax expense (benefit)	2,458		933		1,095
Total deferred income tax expense (benefit)	2,458		2,555		11,346
Total income tax expense (benefit)	$	4,866	$	8,372	$	45,532

Total income taxes differed from the amounts computed by applying the statutory income tax rate to income (loss) before income taxes. The sources of these differences are as follows:



	Year Ended December 31,
thousands except percentages	2017			2016			2015
Income (loss) before income taxes	$	578,068		$	605,352		$	56,630
Statutory tax rate	—		%	—		%	—		%
Tax computed at statutory rate	$	—		$	—		$	—
Adjustments resulting from:
Federal taxes on income attributable to Anadarko’s investment in WES	—			6,162			42,823
State taxes on income attributable to Anadarko’s investment in WES (net of federal benefit)	—			117			298
Texas margin tax expense (benefit) ⁽¹⁾	4,866			2,093			2,411
Income tax expense (benefit)	$	4,866		$	8,372		$	45,532
Effective tax rate	1		%	1		%	80		%


⁽¹⁾	Includes a reduction of $2.2 million in deferred state income taxes for the year ended December 31, 2015. Texas House Bill 32, signed into law in June 2015, reduced the Texas margin tax rates by 0.25%. The law became effective January 1, 2016. WGP is required to include the impact of the law change on its deferred state income taxes in the period enacted.

The tax effects of temporary differences that give rise to significant portions of deferred tax assets (liabilities) are as follows:



	December 31,
thousands	2017			2016
Depreciable property	$	(7,676	)	$	(4,976	)
Credit carryforwards	448			498
Other intangible assets	(189		)	(1,928		)
Other	8			4
Net long-term deferred income tax liabilities	$	(7,409	)	$	(6,402	)

Credit carryforwards, which are available for use on future income tax returns, consist of $0.4 million of state income tax credits that expire in 2026.