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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)

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