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Capital Structure - Changes in Common Shares issued and Outstanding (Details) - shares
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Capital Structure        
Preferred stock, shares authorized (in shares) 10,000,000 10,000,000 10,000,000 10,000,000
Common stock, shares authorized (in shares) 500,000,000 500,000,000 500,000,000 500,000,000
Increase (Decrease) in Stockholders' Equity (in shares)        
Beginning balance, common shares issued (in shares)   417,583,825    
Beginning balance, treasury shares(in shares)   (102,140,814)    
Beginning balance, common shares outstanding (in shares)   315,443,011    
Ending balance, common shares issued (in shares)   418,323,134 417,583,825  
Ending balance, treasury shares (in shares)   (101,580,045) (102,140,814)  
Ending balance, common shares outstanding (in shares)   316,743,089 315,443,011  
Common Stock        
Increase (Decrease) in Stockholders' Equity (in shares)        
Beginning balance, common shares issued (in shares)   417,583,825 416,939,950 415,506,176
Beginning balance, common shares outstanding (in shares)   315,443,011 314,190,042 336,662,624
Shares issued under ESPP   560,769 609,094 283,139
Shares issued from LTIP   739,309 643,875 1,433,774
Share repurchases       (24,189,495)
Ending balance, common shares issued (in shares) 415,506,176 418,323,134 417,583,825 416,939,950
Ending balance, common shares outstanding (in shares) 336,662,624 316,743,089 315,443,011 314,190,042
Treasury Stock        
Increase (Decrease) in Stockholders' Equity (in shares)        
Beginning balance, treasury shares(in shares)   (102,140,814) (102,749,908) (78,843,552)
Shares issued under ESPP   560,769 609,094 283,139
Shares issued from LTIP   0 0 0
Share repurchases       (24,189,495)
Ending balance, treasury shares (in shares) (78,843,552) (101,580,045) (102,140,814) (102,749,908)