XML 83 R70.htm IDEA: XBRL DOCUMENT v3.8.0.1
Equity - Share Repurchase Plan, Preferred Stock (Details)
$ / shares in Units, $ in Millions
 12 Months Ended 288 Months Ended
Oct. 20, 2015
shares
Dec. 31, 2017
USD ($)
item
$ / shares
shares
Dec. 31, 2016
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Share Repurchase Plan          
Increase in authorized number of shares to be repurchased 7,500,000        
Repurchase of common shares (in shares)   300,000 0 6,200,000 22,500,000
Average costs per share | $ / shares   $ 74.27   $ 57.39 $ 31.58
Value of shares repurchased | $   $ 25.0   $ 355.5  
Remaining number of common stock authorized for repurchase under stock repurchase program (in shares)   8,100,000     8,100,000
Percent of outstanding stock authorized for repurchase   11.00%     11.00%
Preferred Stock          
Preferred stock, authorized shares   5,000,000 5,000,000   5,000,000
Preferred stock, par value (in dollars per share) | $ / shares   $ 0.001 $ 0.001   $ 0.001
Preferred stock, issued shares   0 0   0
Preferred stock, outstanding shares   0 0   0
Minimum number of series in which preferred shares may be issued | item   1