Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Property and Equipment
Proved oil and gas properties	$ 4,343,867	$ 3,835,173
Unproved oil and gas properties	84,146	78,429
Furniture, fixtures, and other equipment	38,832	37,505
Less - Accumulated depreciation, depletion, and amortization	(2,599,079)	(2,378,262)
Property and Equipment, Net	1,867,766	1,572,845
Accounts Payable and Accrued Liabilities
Trade accounts payable	42,080	22,459
New Zealand deferred revenue	0	10,000
Accrued operating expenses	15,833	11,044
Accrued payroll costs	14,345	14,298
Asset retirement obligation - current portion	9,279	8,708
Accrued taxes	7,604	7,198
Other payables	6,825	1,887
Total accounts payable and accrued liabilities	95,966	75,594
Accumulated other comprehensive income and related tax effects for 2011
Other comprehensive gain (loss) at December 31, 2010, Gross Value	(218)
Other comprehensive gain (loss) at December 31, 2010, Tax Effect	80
Other comprehensive gain (loss) at December 31, 2010, Net of Tax Value	(138)
Change in fair value of cash flow hedges, Gross Value	(520)
Change in fair value of cash flow hedges, Tax Effect	190
Change in fair value of cash flow hedges, Net of Tax Value	(330)
Effect of cash flow hedges settled during the period, Gross Value	738
Effect of cash flow hedges settled during the period, Tax Effect	(270)
Effect of cash flow hedges settled during the period, Net of Tax Value	468
Other comprehensive gain (loss) at December 31, 2011, Gross Value	0	(218)
Other comprehensive gain (loss) at December 31, 2011, Tax Effect	0	80
Other comprehensive gain (loss) at December 31, 2011, Net of Tax Value	0	(138)
Roll-forward of our asset retirement obligation
Asset Retirement Obligation as of December 31	78,879	64,236	48,785
Accretion expense	4,570	3,956	2,906
Liabilities incurred for new wells and facilities construction	590	1,287	3,400
Reductions due to sold and abandoned wells	(28,194)	(749)	(1,380)
Revisions in estimates	20,548	10,149	10,525
Asset Retirement Obligation as of December 31	$ 76,393	$ 78,879	$ 64,236