Compensation Plans (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Additional disclosures
Performance period to receive awards (in years)	3 years
Stock-based compensation expense	$ 4,350,000	$ 5,551,000
Cash Bonus Plan
Additional disclosures
Amount paid for cash bonuses	24,000,000	21,600,000
Cash bonus expense allocated to general, administrative and exploration expense	4,700,000	3,800,000
Net Profits Plan [Member]
Net Profits Plan
Cash payments made under the Net Profits Plan related to divested properties	286,000	4,300,000
Performance Share Awards Under the Equity Incentive Compensation Plan
Additional disclosures
Performance period to receive awards (in years)	3
Stock-based compensation expense	2,900,000	4,300,000
Unrecognized stock based compensation expense	23,300,000
Restricted Stock Units Under the Equity Incentive Compensation Plan
Additional disclosures
Number of Common Shares to be Received Upon Settlement	1
Stock-based compensation expense	1,200,000	1,100,000
Unrecognized stock based compensation expense	6,500,000
Stock Option Grants Under Prior Stock Option Plans
Stock option activity
Stock options outstanding at the beginning of the period (in shares)	508,214
Exercised (in shares)	(85,303)
Forfeited (in shares)	0
Stock options outstanding at the end of the period (in shares)	422,911
Stock options vested and exercisable at the end of the period (in shares)	422,911
Weighted-Average Exercise Price
Stock options outstanding at the beginning of the period (in dollars per share)	$ 13.86
Exercised (in dollars per share)	$ 12.33
Forfeited (in dollars per share)	$ 0
Stock options outstanding at the end of the period (in dollars per share)	$ 14.16
Stock options vested and exercisable at the end of the period (in dollars per share)	$ 14.16
Aggregate Intrinsic Value
Outstanding at the beginning of the period	30,109,000
Exercised	5,634,000
Outstanding at the end of the period	23,940,000
Vested at the end of the period	23,940,000
Net Profits Plan [Member]
Net Profits Plan
General and administrative expense	4,412,000	5,330,000
Exploration expense	525,000	477,000
Total amount of cash payments made or accrued	$ 4,937,000	$ 5,807,000