Contingencies Relating to Production Leases and Exploration Permits - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 31, 2015	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2012
Loss Contingencies [Line Items]
General and administrative		$ 2,503,000	$ 2,539,000	$ 8,247,000	$ 9,662,000
Bulgaria [Member]
Loss Contingencies [Line Items]
Recovery of contractual obligations						$ 2,000,000
Liquidated damages, claim amount	$ 200,000
Bulgaria [Member] | Aglen Exploration Permit Work Program [Member]
Loss Contingencies [Line Items]
General and administrative						$ 2,000,000