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Compensation Plans (Details) (USD $)
1 Months Ended 3 Months Ended 9 Months Ended
Jul. 27, 2012
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 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   4,300,000 3,800,000 13,600,000 11,300,000  
Net Profits Plan [Member]
           
Net Profits Plan            
General and administrative expense       12,177,000 14,820,000  
Exploration expense       1,421,000 1,569,000  
Total amount of cash payments made or accrued       13,598,000 16,389,000  
Performance Share Awards Under the Equity Incentive Compensation Plan
           
RSUs, PSUs, and Director's Shares [Abstract]            
Stock-based compensation expense   5,100,000 5,900,000 13,200,000 14,300,000  
Unrecognized stock based compensation expense   25,200,000   25,200,000    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Total Fair Value 16,400,000          
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    
LTIP Activity [Abstract]            
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number   692,289   692,289   885,894
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 314,853     314,853    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period       (473,750)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period       (34,708)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value   $ 63.90   $ 63.90   $ 57.52
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value       $ 51.98    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value       $ 43.96    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period, Weighted Average Grant Date Fair Value   $ 65.22   $ 65.22    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Multiplier Assumed       1    
Shares Issued to the Board of Directors [Member]
           
RSUs, PSUs, and Director's Shares [Abstract]            
Stock-based compensation expense   147,000 0 1,337,000 1,042,000  
Stock Issued During Period, Shares, Share-based Compensation, Gross       30,486 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   3,900,000 1,600,000 6,500,000 3,600,000  
Unrecognized stock based compensation expense   17,500,000   17,500,000    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Total Fair Value 18,200,000          
Shares Issued, Net of Shares for Tax Withholdings 111,452          
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period 160,989          
Share-based Compensation, Shares Withheld for Taxes 49,537          
LTIP Activity [Abstract]            
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number   494,913   494,913   308,877
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period       369,332    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period       (160,991)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period       (22,305)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value   $ 51.75   $ 51.75   $ 44.33
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value       $ 49.32    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value       $ 32.02    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period, Weighted Average Grant Date Fair Value   $ 51.06   $ 51.06    
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)       (151,083)    
Forfeited (in shares)       0    
Stock options outstanding at the end of the period (in shares)   357,131   357,131    
Stock options vested and exercisable at the end of the period (in shares)   357,131   357,131    
Weighted-Average Exercise Price            
Stock options outstanding at the beginning of the period (in dollars per share)       $ 13.86    
Exercised (in dollars per share)   $ 12.39   $ 12.39    
Forfeited (in dollars per share)   $ 0   $ 0    
Stock options outstanding at the end of the period (in dollars per share)   $ 14.48   $ 14.48    
Stock options vested and exercisable at the end of the period (in dollars per share)   $ 14.48   $ 14.48    
Aggregate Intrinsic Value            
Outstanding at the beginning of the period       30,109,000    
Exercised       8,308,000    
Outstanding at the end of the period   14,155,000   14,155,000    
Vested at the end of the period   14,155,000   14,155,000    
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   4,083,000 4,229,000      
Exploration expense   403,000 507,000      
Total amount of cash payments made or accrued   4,486,000 4,736,000      
Cash payments made under the Net Profits Plan related to divested properties   $ 274,000 $ 0 $ 2,000,000 $ 6,300,000