Commitments And Contingencies (Narrative) (Detail) a in Millions		1 Months Ended	3 Months Ended										12 Months Ended			50 Months Ended
	Mar. 14, 2016 USD ($)	Nov. 30, 2006 USD ($) item	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 USD ($) a item $ / Boe	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) $ / Boe	Dec. 31, 2010 km²	Dec. 31, 2012 USD ($)
Commitments And Contingencies [Line Items]
Expected settlement of litigation				$ 550,000									$ 550,000
Litigation, accrued expected recoveries				0									$ 0
Period of Charter													2 years
Percentage of share in charter payment													28.10%
Liabilities, guarantees' fair value				1,000,000				$ 1,000,000					$ 1,000,000	$ 1,000,000
Charter fee for production up to 20,000 BOPD | $ / Boe													0.93		0.25
Charter fee for those bbls produced in exess of 20,000 BOPD | $ / Boe													2.50
Company's share of charter expense													$ 10,900,000	11,800,000	$ 10,400,000
Rent expense, operating leases													4,700,000	4,000,000	4,100,000
Abandonment funding				5,137,000				3,537,000					5,137,000	3,537,000
Allowance for accounts with partners								7,600,000						7,600,000
Accounts with partners, net of allowance				27,856,000				10,903,000					27,856,000	10,903,000
Recorded restricted cash				15,830,000				20,830,000					15,830,000	20,830,000
Exploration expense													46,453,000	15,358,000	23,928,000
Dry hole costs and impairment loss on unproved leasehold					$ 9,000,000		$ 24,500,000			$ 2,000,000	$ 9,700,000		45,652,000	13,273,000	22,490,000
Asset retirement obligations				16,166,000				14,846,000				$ 11,464,000	16,166,000	14,846,000	11,464,000		$ 10,368,000
Impairment of Oil and Gas Properties				8,800,000	$ 0	$ 600,000	$ 2,700,000	$ 2,300,000	$ 0	$ 800,000	$ 800,000
Inventory write-off				3,400,000
Gabon Taxation Department [Member]
Commitments And Contingencies [Line Items]
Tax examination settlement payment				300,000									300,000
Sonangol P&P [Member]
Commitments And Contingencies [Line Items]
Accounts with partners, net of allowance				8,100,000									$ 8,100,000
Scenario, Forecast [Member] | Sonangol P&P [Member]
Commitments And Contingencies [Line Items]
Reversal of allowance			$ 7,600,000
Interst income			$ 3,200,000
Gabon [Member]
Commitments And Contingencies [Line Items]
Discount													25.00%
Contractual Obligation Company Share													$ 2,300,000	$ 3,300,000	$ 3,000,000
Contractual Obligation accrued amount				1,800,000									1,800,000
Offshore Gabon [Member]
Commitments And Contingencies [Line Items]
Drilling rig cost day rate													172,000
Remaining commitment of drilling activity				32,200,000									32,200,000
Remaining commitment of drilling activity, net				9,800,000									$ 9,800,000
Percentage of annual funding over seven years													12.14%
Funding period for initial rate													7 years
Second funding period of production license													3 years
Percentage of annual funding over last three years													5.00%
Abandonment cost related to annual funding													$ 61,100,000
Abandonment cost related to annual funding, net													$ 17,300,000
Funding period abandonment funding													10 years
Abandonment funding, gross				18,300,000									$ 18,300,000
Abandonment funding				5,100,000									$ 5,100,000
Number of contract extension periods | item													2
Production license agreement term extended by government													5 years
Angola [Member]
Commitments And Contingencies [Line Items]
Cost related to drilling		$ 29,500,000
Drilling cost to company		$ 14,800,000
Area under acquire property exploration rights agreement term (acres) | a													1.4
Additional joint operation agreement related to third party in working interest percentage													10.00%
Production license agreement term													4 years
Extended drilling period													1 year
Number of additional exploration wells due to exploration period extension | item													2
Production license agreement term extended by government													3 years
Recorded restricted cash				15,000,000									$ 15,000,000
Working interest ownership, percentage		40.00%
Impairment of Oil and Gas Properties													8,200,000
Inventory write-off													1,900,000
Number of wells drilling commitment | item		2
Working interest ownership other, percentage												40.00%
Drilling commitment, gross				10,000,000									10,000,000
Drilling commitment, net				$ 5,000,000									$ 5,000,000
Angola [Member] | Seismic Obligation [Member]
Commitments And Contingencies [Line Items]
Length of acquired property | km²																1,000
Subsequent Event [Member] | Sonangol P&P [Member]
Commitments And Contingencies [Line Items]
Interest receivable	$ 3,200,000
Accounts with partners, net of allowance	7,600,000
Joint interest owner payments	$ 19,000,000