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Incentive Plans (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended
Mar. 09, 2018
Mar. 01, 2018
Feb. 20, 2018
Feb. 06, 2018
Feb. 28, 2018
Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Threshold of deductible compensation for executive officers           $ 1.0
2012 Long-Term Plan [Member]            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Long term plan expiration year           2022
Maximum number of shares available for issuance           8,700,000
Maximum number of shares available for issuance, annual rate           1.00%
Performance-Based RSUs [Member]            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Grants in period           905,500
Stock-based compensation, fair value assumption model           Monte Carlo valuation model
Performance-Based RSUs [Member] | Minimum [Member]            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Fair value per share at grant date           $ 28.27
Performance-Based RSUs [Member] | Maximum [Member]            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Fair value per share at grant date           $ 30.56
Time-Based RSUs [Member]            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Grants in period           736,000
Fair value per share at grant date   $ 25.69 $ 26.56 $ 28.42    
Time-Based RSUs [Member] | Director [Member] | Non-Employee Director Plan [Member]            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Grants in period         77,803  
Fair value per share at grant date         $ 28.28  
Vesting scheme           These units are scheduled to vest on the third anniversary of the date of grant.
Cash-Settled RSUs [Member]            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Grants in period 29,000     715,100