GEOGRAPHICAL SEGMENTS (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
GEOGRAPHICAL SEGMENTS
Number of industry segments	1		1
Number of reportable geographical segments			3
Segment Reporting Information
Oil, natural gas, and natural gas liquids sales	$ 237,848,000	$ 207,954,000	$ 440,419,000	$ 429,683,000
Costs and expenses:
Lease operating expenses	32,463,000	28,422,000	64,320,000	57,524,000
Production and property taxes	13,278,000	12,487,000	25,487,000	23,915,000
Transportation and processing costs	7,805,000	6,020,000	15,081,000	10,879,000
Depletion	71,146,000	60,135,000	136,545,000	109,762,000
Accretion of asset retirement obligations	1,787,000	1,909,000	3,511,000	3,748,000
Segment earnings (loss)	111,369,000	98,981,000	195,475,000	223,855,000
Capital expenditures	358,428,000	217,501,000	626,666,000	492,752,000
Goodwill	257,386,000	255,696,000	257,386,000	255,696,000	256,842,000
Long-lived assets	3,107,670,000	2,517,620,000	3,107,670,000	2,517,620,000
TOTAL ASSETS	4,377,713,000	3,650,776,000	4,377,713,000	3,650,776,000	3,785,388,000
Changes in estimated discounted asset retirement obligations	1,800,000	6,800,000	1,900,000	(2,800,000)
Reconciliation of segment earnings to consolidated earnings before income taxes
Segment earnings	111,369,000	98,981,000	195,475,000	223,855,000
Interest and other income	286,000	141,000	850,000	277,000
General and administrative expense	(15,737,000)	(17,781,000)	(34,771,000)	(36,534,000)
Depreciation and amortization expense	(2,176,000)	(2,311,000)	(4,340,000)	(4,996,000)
Interest expense	(38,842,000)	(37,109,000)	(76,722,000)	(75,152,000)
Realized and unrealized losses on derivative instruments, net	46,047,000	25,031,000	9,801,000	118,242,000
Other, net	(10,265,000)	(12,640,000)	(4,658,000)	(2,807,000)
Earnings before income taxes	90,682,000	54,312,000	85,635,000	222,885,000
United States
Segment Reporting Information
Oil, natural gas, and natural gas liquids sales	186,593,000	169,699,000	352,903,000	354,021,000
Costs and expenses:
Lease operating expenses	23,483,000	22,479,000	47,113,000	45,947,000
Production and property taxes	12,655,000	11,862,000	24,261,000	22,668,000
Transportation and processing costs	3,415,000	3,271,000	7,066,000	5,926,000
Depletion	50,545,000	43,919,000	97,349,000	80,430,000
Accretion of asset retirement obligations	1,480,000	1,627,000	2,903,000	3,176,000
Segment earnings (loss)	95,015,000	86,541,000	174,211,000	195,874,000
Capital expenditures	246,289,000	164,270,000	436,067,000	330,766,000
Goodwill	239,420,000	239,420,000	239,420,000	239,420,000
Long-lived assets	2,196,410,000	1,879,893,000	2,196,410,000	1,879,893,000
TOTAL ASSETS	3,387,568,000	2,938,736,000	3,387,568,000	2,938,736,000
Reconciliation of segment earnings to consolidated earnings before income taxes
Segment earnings	95,015,000	86,541,000	174,211,000	195,874,000
Canada
Segment Reporting Information
Oil, natural gas, and natural gas liquids sales	51,255,000	38,255,000	87,516,000	75,662,000
Costs and expenses:
Lease operating expenses	8,980,000	5,943,000	17,207,000	11,577,000
Production and property taxes	623,000	625,000	1,226,000	1,247,000
Transportation and processing costs	4,390,000	2,749,000	8,015,000	4,953,000
Depletion	20,601,000	16,216,000	39,196,000	29,332,000
Accretion of asset retirement obligations	279,000	258,000	554,000	523,000
Segment earnings (loss)	16,382,000	12,464,000	21,318,000	28,030,000
Capital expenditures	108,724,000	52,240,000	186,667,000	159,881,000
Goodwill	17,966,000	16,276,000	17,966,000	16,276,000
Long-lived assets	815,794,000	548,632,000	815,794,000	548,632,000
TOTAL ASSETS	893,797,000	622,162,000	893,797,000	622,162,000
Reconciliation of segment earnings to consolidated earnings before income taxes
Segment earnings	16,382,000	12,464,000	21,318,000	28,030,000
International
Costs and expenses:
Accretion of asset retirement obligations	28,000	24,000	54,000	49,000
Segment earnings (loss)	(28,000)	(24,000)	(54,000)	(49,000)
Capital expenditures	3,415,000	991,000	3,932,000	2,105,000
Long-lived assets	95,466,000	89,095,000	95,466,000	89,095,000
TOTAL ASSETS	96,348,000	89,878,000	96,348,000	89,878,000
Reconciliation of segment earnings to consolidated earnings before income taxes
Segment earnings	$ (28,000)	$ (24,000)	$ (54,000)	$ (49,000)