Commitments and Contingencies - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Additional Textual [Abstract]
Remaining environmental accrued liability recorded	$ 0	$ 0
Beta's decommissioning obligations, supported by surety bonds	161,300,000
Beta's decommissioning obligations, cash	300,000
Commitment fee expense	1,000,000.0
Commitment fee expense accrual	$ 1,200,000

Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable in twelve months or in the next operating cycle if longer.

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