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Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Deferred tax liabilities |  |
Investments | $ 112,343 | $ 171,517
Derivative contracts | 1,128 | 0
Total deferred tax liabilities | 113,471 | 171,517
Deferred tax assets |  |
Property, plant and equipment | 267,865 | 391,273
Derivative contracts | 0 | 3,131
Net operating loss carryforwards | 302,190 | 217,259
Tax credits and other carryforwards | 35,640 | 33,001
Asset retirement obligations | 15,016 | 18,843
Other | 3,816 | 8,959
Total deferred tax assets | 624,527 | 672,466
Valuation allowance | (511,056) | (500,949)
Net deferred tax liability | $ 0 | $ 0

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