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Subsequent Events (Details) - USD ($)
$ in Millions
Oct. 18, 2018
Sep. 30, 2018
Feb. 28, 2018
Dec. 31, 2017
Laramie Energy Company        
Subsequent Event [Line Items]        
Ownership interest in Laramie Energy, LLC   39.10% 39.10% 42.30%
Laramie Energy Company | Subsequent Event        
Subsequent Event [Line Items]        
Ownership interest in Laramie Energy, LLC 46.00%      
Capital Unit, Class A | Laramie Energy Company | Subsequent Event        
Subsequent Event [Line Items]        
Stock repurchased during period (in shares) 138,795      
Stock repurchased during period $ 14.8