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Investment in Laramie Energy - Change in Equity Investment (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]      
Beginning balance $ 76,203    
Equity earnings (loss) from Laramie Energy (22,381) $ (55,983) $ 2,849
Ending balance 108,823 76,203  
Laramie Energy Company      
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]      
Beginning balance 76,203 104,657 101,796
Equity earnings (loss) from Laramie Energy (28,198) (15,713) 2,278
Accretion of basis difference 5,818 811 571
Impairment 0 (41,081) 0
Investments 55,000 27,529 12
Ending balance $ 108,823 $ 76,203 $ 104,657