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Acquisition - Additional Information (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Jul. 27, 2017
Feb. 10, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2017
Business Acquisition [Line Items]              
Operating earnings     $ 105,938 $ 98,624 $ 310,582 $ 281,916  
Purchase price paid for business acquisition         36,761    
Goodwill     205,211   $ 205,211   $ 198,370
Fairborn Acquisition [Member]              
Business Acquisition [Line Items]              
Business acquisition, date of acquisition         Feb. 10, 2017    
Business acquisition purchase price   $ 400,500          
Other assets   2,820          
Asset retirement obligation   2,820          
Provisional other assets recorded previously   4,000          
Provisional asset retirement obligation recorded previously   4,000          
Depreciation and amortization         $ 11,200    
Operating earnings     $ 6,547   20,380    
Direct acquisition cost             $ 5,500
Property and equipment   314,897          
Goodwill   $ 64,485          
Fairborn Acquisition [Member] | Fair Value Adjustment to Inventory [Member]              
Business Acquisition [Line Items]              
Operating earnings         $ 600    
Wildcat Minerals LLC [Member]              
Business Acquisition [Line Items]              
Business acquisition, effective date of acquisition         Jul. 27, 2017    
Purchase price paid for business acquisition $ 36,800            
Current assets 3,100            
Property and equipment 28,300            
Intangible and other assets 1,400            
Current liabilities 2,800            
Goodwill $ 6,800