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Commitments and Contingencies - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Commitments and Contingencies Disclosure [Line Items]      
Letters of credit, collateral for retrospective premiums and retained losses $ 60,800,000   $ 60,800,000
Commitments to purchase major equipment and investments 11,700,000   $ 11,700,000
Agreement expire description     The agreements expire in years 2020 through 2022.
Purchase obligations for remainder of 2020 13,000,000.0   $ 13,000,000.0
Purchase obligations for 2021 10,000,000.0   10,000,000.0
Purchase obligations for 2022 3,400,000   3,400,000
Current obligation 26,400,000   26,400,000
Deposit revaluation charge 9,200,000 $ 12,700,000  
Capacity reservation agreement balance 0    
Letter of Credit      
Commitments and Contingencies Disclosure [Line Items]      
Amount drawn under letters of credit $ 0   $ 0