INCOME TAXES

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The Tax Cuts and Jobs Act (the "Act") was enacted on December 22, 2017. The Act reduces the U.S. federal corporate tax rate from the previous rate of 35 percent to 21 percent, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred, and creates new taxes on certain foreign sourced earnings. At December 31, 2017, the Company has calculated the accounting for the tax effects of enactment of the Act as written, and made a reasonable estimate of the effects on the existing deferred tax balances. The Company will continue to refine the calculations as additional analysis is completed. In addition, these estimates may also be affected as the Company gains a more thorough understanding of the tax law, including those related to the deductibility of purchased assets, state tax treatment, and amounts related to Employee compensation. This re-measurement in 2017 resulted in a reduction in the Company's net deferred tax liability, as noted by the change in federal statutory tax rate as noted in the provision for income taxes below. The components of deferred tax assets and liabilities at December 31, 2017 and 2016, are as follows:



(in millions)	2017		2016
DEFERRED TAX LIABILITIES:
Accelerated depreciation	$	3,193	$	4,726
Other	86		134
Total deferred tax liabilities	3,279		4,860
DEFERRED TAX ASSETS:
Fuel derivative instruments	12		233
Construction obligation	326		402
Accrued employee benefits	309		451
Other	274		400
Total deferred tax assets	921		1,486
Net deferred tax liability	$	2,358	$	3,374

The provision for income taxes is composed of the following:



(in millions)	2017			2016		2015
CURRENT:
Federal	$	904		$	778	$	1,292
State	72			69		114
Total current	976			847		1,406
DEFERRED:
Federal	192			426		(97		)
State	5			30		(11		)
Change in federal statutory tax rate	(1,410		)	—		—
Total deferred	(1,213		)	456		(108		)
	$	(237	)	$	1,303	$	1,298

The effective tax rate on income before income taxes differed from the federal income tax statutory rate for the following reasons:



(in millions)	2017			2016			2015
Tax at statutory U.S. tax rates	$	1,138		$	1,241		$	1,218
State income taxes, net of federal benefit	50			64			66
Change in federal statutory tax rate	(1,410		)	—			—
Other, net	(15		)	(2		)	14
Total income tax provision	$	(237	)	$	1,303		$	1,298

The only periods subject to examination for the Company’s federal tax return are the 2016 and 2017 tax years.