Income taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Components of pre-tax book income (loss)

The components of pre-tax book income (loss) consist of the following:
	Year Ended December 31,
	2017		2016			2015
U.S.	$	26,823	$	(86,545	)	$	(62,203	)
Foreign	28,548		(164,320		)	21,081
Total	$	55,371	$	(250,865	)	$	(41,122	)

Significant components of the income before income tax expense

Significant components of the income tax expense consist of the following:
	Year Ended December 31,
	2017			2016			2015
Current:
U.S. federal current expense (benefit)	$	(106	)	$	(231	)	$	—
State current expense (benefit)	1,244			(130		)	(706		)
Foreign current expense	12,305			5,726			13,473
Total current expense	13,443			5,365			12,767
Deferred:
U.S. federal deferred benefit	(2,522		)	(1,564		)	(1,564		)
State deferred benefit	(14		)	—			—
Foreign deferred tax expense	(3,324		)	(977		)	(1,927		)
Total deferred benefit	(5,860		)	(2,541		)	(3,491		)
Total income tax expense	$	7,583		$	2,824		$	9,276

Reconciliation of the statutory U.S. Federal income tax rate to the effective income tax rate on income (loss)

A reconciliation of the statutory U.S. Federal income tax rate to the effective income tax rate on income (loss) is as follows:
	2017		2016		2015
Federal Statutory Rate	35.0	%	35.0	%	35.0	%
Permanent differences	57.5		7.7		1.9
State taxes, net of Federal benefit	(6.6	)	6.1		(16.0	)
Rate change	370.5		(4.2	)	—
Foreign earnings taxed at different rates than U.S.	(40.5	)	(13.5	)	3.0
Valuation allowance	(401.4	)	(27.5	)	(56.6	)
Changes in uncertain tax reserves	3.8		(1.0	)	(4.2	)
Other	(4.6	)	(3.7	)	14.3
Effective tax rate	13.7	%	(1.1	)%	(22.6	)%

Significant components of deferred tax assets and liabilities

The significant components of our deferred tax assets and liabilities as of December 31 are as follows:  

	2017			2016
Deferred tax assets:
Accrued postretirement benefit cost	$	39,417		$	69,725
Accrued liabilities	3,125			4,679
Share-based compensation	1,319			6,071
Goodwill	1,996			5,539
Net operating losses and tax credits	551,098			739,712
Foreign basis differences	13,875			13,929
Ravenswood retiree legal settlement	3,089			8,683
Other	3,091			5,747
Total deferred tax assets	617,010			854,085
Valuation allowance	(607,813		)	(839,082		)
Net deferred tax assets	$	9,197		$	15,003
Deferred tax liabilities:
Tax over financial statement depreciation	$	(109,733	)	$	(119,378	)
Unremitted foreign earnings	—			(808		)
Total deferred tax liabilities	(109,733		)	(120,186		)
Net deferred tax liability	$	(100,536	)	$	(105,183	)

Changes in valuation allowance

The changes in the valuation allowance are as follows:

	2017			2016			2015
Beginning balance, valuation allowance	$	839,082		$	768,764		$	748,283
Remeasurement of deferred tax assets	(205,150		)	—			—
Release of valuation allowance	—			(6,007		)	—
Other change in valuation allowance	(26,119		)	76,325			20,481
Ending balance, valuation allowance	$	607,813		$	839,082		$	768,764

Significant components of Net Operating Loss Carryforwards

The significant components of our net operating loss carryforwards ("NOLs") are as follows:

	2017		2016
Federal (1)	$	1,514,243	$	1,510,558
State (2)	2,480,523		1,901,554
Foreign (3)	532,582		540,819

⁽¹⁾  The federal NOL begins to expire in 2028.

⁽²⁾  The state NOLs begin to expire in 2027.

⁽³⁾  The Icelandic NOL begins to expire in 2017; the Netherlands NOL begins to expire in 2022.

Reconciliation of beginning and ending amounts of gross unrecognized tax benefits

A reconciliation of the beginning and ending amounts of gross unrecognized tax positions (excluding interest) is as follows:

	2017			2016			2015
Balance as of January 1,	$	6,400		$	3,800		$	2,000
Additions based on tax positions related to the current year	2,100			2,700			1,800
Decreases due to lapse of applicable statute of limitations	(100		)	(100		)	—
Settlements	—			—			—
Balance as of December 31,	$	8,400		$	6,400		$	3,800

Significant components of unrecognized tax positions

The components of our unrecognized tax positions are as follows:

	2017		2016		2015
Highly certain tax positions	$	8,300	$	6,300	$	3,700
Other unrecognized tax benefits	100		100		100
Gross unrecognized tax benefits	$	8,400	$	6,400	$	3,800
Accrued interest and penalties related to unrecognized tax positions	$	40	$	—	$	—