v2.4.0.6
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