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Commitments and Contingencies - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Jul. 18, 2018
Jun. 30, 2019
Sep. 30, 2019
Commitments and Contingencies Disclosure [Line Items]      
Letters of credit, collateral for retrospective premiums and retained losses     $ 63,400,000
Commitments to purchase major equipment and investments     $ 63,300,000
Agreement expire description     The agreements expire in years 2019 through 2023.
Current obligation     $ 44,900,000
Purchase obligations for remainder of 2019     7,200,000
Deposit revaluation charge   $ 12,700,000  
Aggregate proposed penalty in connection with accident in site $ 74,000    
Letter of Credit      
Commitments and Contingencies Disclosure [Line Items]      
Amount drawn under letters of credit     $ 0