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Equity - Share Repurchase Plan, Preferred Stock (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Oct. 23, 2018
shares
Oct. 20, 2015
shares
Dec. 31, 2018
USD ($)
item
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
shares
Share Repurchase Plan          
Increase in authorized number of shares to be repurchased 5,000,000 7,500,000      
Repurchase of common shares (in shares)     6,100,000 300,000 0
Average costs per share | $ / shares     $ 79.94 $ 74.27  
Value of shares repurchased | $     $ 484.9 $ 25.0  
Remaining number of common stock authorized for repurchase under stock repurchase program (in shares)     7,000,000    
Percent of outstanding stock authorized for repurchase     11.00%    
Preferred Stock          
Preferred stock, authorized shares     5,000,000 5,000,000  
Preferred stock, par value (in dollars per share) | $ / shares     $ 0.001 $ 0.001  
Preferred stock, issued shares     0 0  
Preferred stock, outstanding shares     0 0  
Minimum number of series in which preferred shares may be issued | item     1