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Stock-Based Compensation - Narrative (Details)
1 Months Ended 12 Months Ended
Feb. 22, 2016
USD ($)
shares
Feb. 19, 2016
performance_criteria
Jan. 05, 2016
USD ($)
shares
Feb. 28, 2015
Aug. 31, 2014
USD ($)
Sep. 30, 2013
USD ($)
Feb. 28, 2013
USD ($)
Dec. 31, 2015
USD ($)
Award_Type
shares
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Oct. 01, 2013
$ / shares
Aug. 27, 2013
USD ($)
$ / shares
Feb. 11, 2013
USD ($)
$ / shares
May. 01, 2012
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Share-based compensation expense               $ 13,700,000 $ 21,500,000 $ 51,800,000        
Stock-based compensation tax benefit               0 (1,375,000) 18,913,000        
Defined contribution cost recognized               $ (6,400,000) (4,900,000) 7,400,000        
Deferred Compensation, Share-based Payments                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Common shares held in employee trust earned but not distributed (in shares) | shares               534,174            
Shares issued in period (in shares) | shares               0            
Restricted Stock Awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Share-based compensation expense               $ 400,000 1,000,000 200,000        
Award vesting period               3 years            
Compensation cost not yet recognized               $ 500,000            
Fair value of award               200,000 600,000 1,600,000        
Aggregate intrinsic value               $ 900,000            
Weighted-average remaining contractual term of non-vested shares               1 year 2 months 9 days            
Restricted Stock Awards | Minimum                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Expected forfeiture rate               5.00%            
Restricted Stock Awards | Maximum                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Expected forfeiture rate               6.00%            
Restricted Stock Awards | Graduated or graded vesting | Minimum                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting period               3 years            
Restricted Stock Awards | Graduated or graded vesting | Maximum                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting period               4 years            
Restricted Stock Units                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Share-based compensation expense               $ 1,400,000 1,500,000 1,400,000        
Aggregate intrinsic value               $ 7,500,000            
Stock Appreciation Rights                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Share-based compensation expense                 100,000 300,000        
Compensation cost not yet recognized                 0          
Term of award               7 years            
Aggregate intrinsic value               $ 2,900,000            
Weighted-average remaining contractual term of non-vested shares               2 years 2 months 28 days            
Stock Appreciation Rights | Graduated or graded vesting                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting period               3 years            
Award vesting percentage               33.33%            
Performance Shares                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Share-based compensation expense               $ 18,300,000 20,800,000 30,900,000        
Award vesting period               3 years            
Compensation cost not yet recognized               $ 18,400,000            
Number of performance award types | Award_Type               3            
Period of recognition for unrecognized compensation costs               1 year 15 days            
Aggregate intrinsic value               $ 35,900,000            
Weighted-average remaining contractual term of non-vested shares               1 year 1 month 10 days            
Number of performance awards where the performance period has ended | Award_Type               2            
Performance Shares | Minimum                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Expected forfeiture rate               0.00%            
Performance Shares | Maximum                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Expected forfeiture rate               6.00%            
Internal Metrics Performance Share Awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Number of performance award types | Award_Type               2            
Rights to share portion of award, maximum percent               100.00%            
Market Based Performance Share Awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Number of performance award types | Award_Type               1            
Rights to share portion of award, maximum percent               100.00%            
Rights to cash portion of award, maximum percent               100.00%            
Employee Performance Share Awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting period       3 years       3 years            
Employee Performance Share Awards | Performance metric: average production                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting percentage               33.33%            
Employee Performance Share Awards | Performance metric: average finding costs                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting percentage               33.33%            
Employee Performance Share Awards | Performance metric: average reserve replacement                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting percentage               33.33%            
Hybrid Performance Shares                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Minimum operating cash flow for performance based award               $ 100,000,000            
Hybrid Performance Shares | Graduated or graded vesting                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting period               3 years            
Hybrid Performance Shares | 25% vesting on first and second anniversary                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting percentage               25.00%            
Hybrid Performance Shares | 50% vesting on third anniversary                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Award vesting percentage               50.00%            
TSR Performance Share Awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Performance period of peer group               3 years            
Cash payments for share-based compensation               $ 7,000,000 14,300,000 0        
TSR Performance Share Awards | Liability                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Current portion of share-based liability               1,800,000 7,000,000          
Noncurrent portion of share-based liability               900,000 4,000,000          
SEIP                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Share-based compensation expense               $ (300,000) 3,000,000 $ 11,500,000        
Number of trading days during which the closing price meets or exceeds the goal price               20 days            
Number of consecutive trading days for payout               60 days            
Period for interim goal               2 years            
Period for final goal               4 years            
Percent of employee salary for interim goal               20.00%            
Percent of employee salary for final goal               50.00%            
Percent of employee salary if interim goal is met               30.00%            
Percentage of aggregate distribution to be paid immediately               25.00%            
Percentage of aggregate distribution to be paid on deferred dates               75.00%            
Payments Related to Share Based Compensation                 $ 13,000,000          
SEIP | Percent paid in last 18 months                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Percentage of aggregate distribution to be paid immediately               100.00%            
SEIP III                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Interim goal price (in USD per share) | $ / shares                         $ 25.00 $ 25.00
Final goal price (in USD per share) | $ / shares                       $ 37.50   $ 37.50
Interim goal payable                         $ 6,800,000  
Percent of interim goal paid         75.00%   25.00%              
Interim goal payment         $ 4,900,000   $ 1,700,000              
Final goal payable                       $ 11,100,000    
Percent of final goal paid         75.00% 25.00%                
Final goal payment         $ 8,100,000 $ 2,800,000                
SEIP IV                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Interim goal price (in USD per share) | $ / shares                     $ 55.00      
Final goal price (in USD per share) | $ / shares                     $ 80.00      
Final goal payable               $ 0            
Subsequent Event | Employee Performance Share Awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Number of performance criteria | performance_criteria   3                        
Number of performance awards issued and certified (in shares) | shares 340,960                          
Grant date fair value of performance shares issued and certified $ 9,100,000                          
Subsequent Event | TSR Performance Share Awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Number of performance awards issued and certified (in shares) | shares     254,980                      
Grant date fair value of performance shares issued and certified     $ 5,900,000                      
Cash component of performance share award     $ 1,800,000