Income taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Accrued postretirement benefit cost	$ 14,825	$ 15,683
Accrued liabilities	1,080	4,494
Share-based compensation	6,421	7,350
Derivative and hedging contracts	0	58,687
Goodwill	7,949	10,521
Pension and post-retirement obligations in other comprehensive income	78,753	81,567
Net operating losses and tax credits	700,819	646,158
Foreign basis differences	2,083	668
Other	10,676	8,748
Total deferred tax assets	822,606	833,876
Valuation allowance	(768,764)	(748,283)
Net deferred tax assets	53,842	85,593
Deferred tax liabilities:
Tax over financial statement depreciation	(116,327)	(142,627)
Pension	(15,608)	(14,222)
Unremitted foreign earnings	(18,901)	(30,308)
Total deferred tax liabilities	(150,836)	(187,157)
Net deferred tax liability	$ (96,994)	$ (101,564)

The amount as of the balance sheet date of the estimated future tax effects attributable to the difference between the tax basis and the basis in accordance with generally accepted accounting principles of foreign basis differences which will decrease future taxable income when such basis difference reverses.

No definition available.