INCOME TAXES

The reconciliation of income tax expense with federal income taxes at statutory rate follows (in thousands of dollars):



	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Federal income tax (21% in 2018 and 35% in 2017)	$	35,914		$	87,499		$	168,288		$	254,753
States income taxes, net of federal income tax benefit	4,817			5,996			22,572			17,458
Clean energy credit	(2,897		)	(8,902		)	(13,575		)	(25,917		)
Foreign rate difference	1,895			2,152			8,880			6,265
Impairment and other charges	27,875			—			27,875			19,188
Stock compensation benefit	(8,289		)	(575		)	(18,899		)	(8,314		)
Other, net	(3,343		)	(6,988		)	2,657			3,806
Income tax expense	$	55,972		$	79,182		$	197,798		$	267,239
Effective tax rate	32.7		%	31.7		%	24.7		%	36.7		%

The Tax Cut and Jobs Act (the Tax Act) was enacted on December 22, 2017. The Company applied Staff Accounting Bulletin (SAB) 118 when accounting for the enactment-date effects of the Tax Act at December 31, 2017 and recorded estimates primarily related to the revaluation of deferred tax balances and the one-time transition tax. As of September 30, 2018, the Company had not completed the analysis for all of the tax effects of the Tax Act and had not recorded any additional adjustments to the amounts recorded at December 31, 2017. The Company expects to complete the Tax Act estimates in the fourth quarter of 2018 and does not expect a material impact to the Consolidated Financial Statements.