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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