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Investment in Laramie Energy - Change in Equity Investment (Detail) - USD ($)
$ in Thousands
12 Months Ended
Mar. 01, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]        
Beginning balance   $ 108,823    
Equity earnings (losses) from Laramie Energy, LLC   18,369 $ (22,381) $ (55,983)
Ending balance   127,192 108,823  
Laramie Energy Company        
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]        
Beginning balance   108,823 76,203 104,657
Equity earnings (losses) from Laramie Energy, LLC   13,043 (28,198) (15,713)
Accretion of basis difference   5,326 5,818 811
Impairment   0 0 (41,081)
Investments $ 55,000 0 55,000 27,529
Ending balance   $ 127,192 $ 108,823 $ 76,203