Subsequent Events (Details) (USD $)	1 Months Ended		3 Months Ended		49 Months Ended	50 Months Ended		1 Months Ended		15 Months Ended	1 Months Ended	17 Months Ended	1 Months Ended	6 Months Ended	0 Months Ended
	Nov. 30, 2012	Oct. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Nov. 01, 2012	Jun. 30, 2012 lease_block	Nov. 30, 2012 Subsequent Event [Member]	Sep. 30, 2012 Kaybob Duvernay [Member]	Jun. 30, 2012 Kaybob Duvernay [Member] Alta Energy [Member]	Nov. 30, 2012 Kaybob Duvernay [Member] Subsequent Event [Member] Alta Energy [Member]	Aug. 31, 2012 Kaybob Duvernay [Member] Subsequent Event [Member] Alta Energy [Member]	Oct. 31, 2012 Tuscaloosa Marine Shale [Member]	Sep. 30, 2012 Tuscaloosa Marine Shale [Member] acre	Sep. 28, 2011 Fifty Million Share Repurchase Program [Member]
Subsequent Event [Line Items]
Results of Operations, Dry Hole Costs	$ 4,300,000	$ 33,900,000	$ 43,700,000
Treasury Stock, Shares, Acquired		97,496		243,700	2,300,000	2,400,000
Stock Repurchase Program, Authorized Amount															50,000,000
Treasury Stock Acquired, Average Cost Per Share				$ 55.53	$ 46.67	$ 46.84
Stock options, net-settled					45,000
Treasury Stock, Value, Acquired, Cost Method		5,000,000		13,500,000	105,800,000	110,800,000
Common Stock, Shares, Outstanding			15,292,448		15,292,448	15,194,952	15,292,448
Payments to explore and develop oil and gas properties			17,205,000	11,476,000						12,300,000	800,000	13,100,000	4,300,000	8,800,000
Working Interests in Oil and Gas Properties													25.00%
Gas and oil area undeveloped net (in acres)														24,000
Percentage Investment in Project									5.00%
Exploration expenses			44,832,000	0
Number of lease blocks, oil and gas							6
Leasehold Improvements and Rental Payments, Amount Paid in August 2012								$ 4,300,000