Compensation Plans (Details) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 27, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
RSUs, PSUs, and Director's Shares [Abstract]
Performance period to receive awards (in years)				3 years
Cash Bonus Plan
Cash Bonus [Abstract]
Amount paid for cash bonuses				$ 24,000,000	$ 21,600,000
Cash bonus expense allocated to general, administrative and exploration expense		4,600,000	3,700,000	9,300,000	7,500,000
Net Profits Plan [Member]
Net Profits Plan
General and administrative expense				8,094,000	10,591,000
Exploration expense				1,018,000	1,062,000
Total amount of cash payments made or accrued				9,112,000	11,653,000
Performance Share Awards Under the Equity Incentive Compensation Plan
RSUs, PSUs, and Director's Shares [Abstract]
Stock-based compensation expense		5,200,000	4,100,000	8,100,000	8,400,000
Unrecognized stock based compensation expense		16,500,000		16,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	314,853
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period	609,714
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period, Net of Shares Withheld for Tax	812,562
Share-based Compensation, Shares Withheld for Taxes	406,866
Common stock awards vesting multiplier, low end of range		0		0
Common stock awards vesting multiplier, high end of range		2		2
Performance period to receive awards (in years)				3
Shares Issued to the Board of Directors [Member]
RSUs, PSUs, and Director's Shares [Abstract]
Stock-based compensation expense				1,129,000	1,005,000
Stock Issued During Period, Shares, Share-based Compensation, Gross	3,986			26,500	21,568
Restricted Stock Units Under the Equity Incentive Compensation Plan
RSUs, PSUs, and Director's Shares [Abstract]
Number of Common Shares to be Received Upon Settlement				1
Stock-based compensation expense		1,400,000	985,000	2,600,000	2,000,000
Unrecognized stock based compensation expense		4,900,000		4,900,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	365,787
Shares Issued, Net of Shares for Tax Withholdings	111,123
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period	160,484
Share-based Compensation, Shares Withheld for Taxes	49,361
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)				(108,299)
Forfeited (in shares)				0
Stock options outstanding at the end of the period (in shares)		399,915		399,915
Stock options vested and exercisable at the end of the period (in shares)		399,915		399,915
Weighted-Average Exercise Price
Stock options outstanding at the beginning of the period (in dollars per share)				$ 13.86
Exercised (in dollars per share)		$ 12.36		$ 12.36
Forfeited (in dollars per share)		$ 0		$ 0
Stock options outstanding at the end of the period (in dollars per share)		$ 14.26		$ 14.26
Stock options vested and exercisable at the end of the period (in dollars per share)		$ 14.26		$ 14.26
Aggregate Intrinsic Value
Outstanding at the beginning of the period				30,109,000
Exercised				6,667,000
Outstanding at the end of the period		13,937,000		13,937,000
Vested at the end of the period		13,937,000		13,937,000
Employee Stock Purchase Plan
Employee Stock Purchase Plan [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate		15.00%		15.00%
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription		0		0
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Offering Date				85.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		1,337,000		1,337,000
Stock Issued During Period, Shares, Employee Stock Purchase Plans		37,124	22,373
Net Profits Plan [Member]
Net Profits Plan
General and administrative expense		3,682,000	5,261,000
Exploration expense		493,000	585,000
Total amount of cash payments made or accrued		4,175,000	5,846,000
Cash payments made under the Net Profits Plan related to divested properties		$ 1,400,000	$ 2,000,000	$ 1,700,000	$ 6,300,000