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Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Deferred tax liabilities    
Investments $ 109,289 $ 112,343
Derivative contracts 29 1,128
Total deferred tax liabilities 109,318 113,471
Deferred tax assets    
Property, plant and equipment 300,704 267,865
Net operating loss carryforwards 383,418 302,190
Tax credits and other carryforwards 34,148 35,640
Asset retirement obligations 18,747 15,016
Other 2,290 3,816
Total deferred tax assets 739,307 624,527
Valuation allowance (629,989) (511,056)
Net deferred tax liability $ 0 $ 0