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Stockholders' Equity (Details)
$ / shares in Units, $ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
vote
$ / shares
shares
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
shares
Feb. 08, 2017
shares
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Common stock shares authorized (in shares) 300,000,000.0      
Common stock par value (USD per share) | $ / shares $ 0.01      
Preferred stock authorized shares (in shares) 10,000.00      
Preferred stock par or stated value (in dollars per share) | $ / shares $ 0.01      
Common stock, number of votes granted per common share held | vote 1      
Treasury stock, shares, acquired (in shares) 0   600,000  
Value of treasury stock acquired | $     $ 40.1  
Series A Preferred Stock        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Number of shares called by each right       0.001
Performance-based Restricted Stock Units        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Value of treasury stock acquired | $ $ 3.0 $ 0.3    
Repurchase of shares upon vesting of PRSUs withheld to satisfy tax withholding obligations (in shares) 100,000      
February 2016 Program        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Remaining shares under share repurchase authorization (in shares) 226,900,000