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Investment in Laramie Energy Change in Equity Investment (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]    
Beginning balance $ 108,823  
Equity earnings from Laramie 8,746 $ (1,871)
Ending balance 117,569  
Laramie Energy Company    
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]    
Beginning balance 108,823  
Equity earnings from Laramie 7,415  
Accretion of basis difference 1,331  
Ending balance $ 117,569