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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
May 06, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost related to unvested awards $ 27.4    
Unrecognized compensation cost, expected to be capitalized $ 3.5    
Weighted average years over which this cost will be recognized 1 year    
Increase in stock compensation expense due to the restricted stock awards granted $ 1.1    
Stock Appreciation Rights (SARs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted 0 0  
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized     7,230,000
Incentive Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized     2,000,000
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock options granted 0 0  
Stock Performance Measures [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 0.00%    
Stock Performance Measures [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 200.00%    
Cash flow to total assets performance [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 0.00%    
Cash flow to total assets performance [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 200.00%    
Cash flow to total assets performance [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 100.00%    
Time Vested [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Shares granted 839,498 461,799  
Performance Shares [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted 362,070 152,373  
Performance Shares [Member] | Cash flow to total assets performance [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Cliff vest [Member] | Performance Shares [Member] | Stock Performance Measures [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
2018 [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 27.00%    
2017 [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 93.00%    
2016 [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 164.00%