Commitments and Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) bbl
Commitments and Contingencies Disclosure [Line Items]
Long-term Purchase Commitment, Minimum Volume Required | bbl	8,000
Marketing Agreement	10 years
Operating Leases, Future Minimum Payments, Remainder of Fiscal Year	$ 0.1
Operating Leases, Future Minimum Payments Due, Next Twelve Months	0.4
Operating Leases, Future Minimum Payments, Due in Two Years	0.7
Operating Leases, Future Minimum Payments, Due in Three Years	0.7
Operating Leases, Future Minimum Payments, Due in Four Years	0.7
Operating Leases, Future Minimum Payments, Due in Five Years	0.2
Asset Retirement Obligation	3.8
Legal Reserve
Commitments and Contingencies Disclosure [Line Items]
Reserve established for contingency matters	0.1
Crude Oil Gathering And Transportation Services
Commitments and Contingencies Disclosure [Line Items]
Contractual Obligation, Due in 2018	2.7
Contractual Obligation, Due in 2019	11.7
Contractual Obligation, Due 2020 through 2025	13.0
Contractual Obligation, Due 2026	7.4
Contractual Obligation, Due 2027 through 2030	3.8
Contractual Obligation, Due 2031	2.2
Contract Drilling [Member]
Commitments and Contingencies Disclosure [Line Items]
Contractual Obligation	$ 12.3