Income taxes (Tables)

12 Months Ended

Dec. 31, 2013

Components of income tax expense

Income tax expense consists of the following for the years ended December 31:

	2013			2012			2011
Current tax expense
Federal tax benefit	$	(29	)	$	(23	)	$	(21	)
State tax expense	1,144			141			200
Current tax expense	1,115			118			179
Deferred tax expense
Federal tax expense	30,327			35,579			27,290
State tax expense	1,407			2,140			8,455
Deferred tax expense	31,734			37,719			35,745
	$	32,849		$	37,837		$	35,924

Income tax rate reconciliation

Income tax expense differed from amounts computed by applying the U.S. Federal income tax rate as follows for the years ended December 31:

	2013		2012		2011
Statutory rate	35.0	%	35.0	%	35.0	%
State income taxes, net of federal benefit	3.8	%	2.1	%	3.6	%
Statutory depletion	(0.7	)%	(0.04	)%	(0.5	)%
Valuation allowance	(1.7	)%	—	%	7.3	%
Other	0.7	%	(0.04	)%	0.7	%
Effective tax rate	37.1	%	37.0	%	46.1	%

Components of deferred tax assets and liabilities

Components of the deferred tax assets and liabilities are as follows at December 31:

	2013			2012
Deferred tax assets related to
Asset retirement obligations	$	8,777		$	8,496
Accrued expenses, allowance and other	14,508			10,129
Derivative instruments	129			—
Net operating loss carryforwards
Federal	124,855			153,709
State	13,350			16,339
Statutory depletion carryforwards	2,867			2,242
Alternative minimum tax credit carryforwards	308			308
	164,794			191,223
Less valuation allowance	(10,385		)	(11,858		)
Deferred tax asset	154,409			179,365
Deferred tax liabilities related to
Derivative instruments	—			(32,438		)
Property and equipment	(206,799		)	(181,489		)
Inventories	(1,205		)	(1,211		)
Deferred tax liability	(208,004		)	(215,138		)
Net deferred tax liability	(53,595		)	(35,773		)
Less net current deferred tax asset (liability)	9,260			(26,872		)
Long-term deferred tax liability	$	(62,855	)	$	(8,901	)