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Share-Based Compensation Plans - Relative Total Shareholder Return Performance Unit Activity (Details) - USD ($)
$ / shares in Units, $ in Millions
6 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Jun. 27, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Beginning balance (in shares) 0 0 0  
Granted (in shares) 0 0 0 0
Ending balance (in shares) 0 0 0 0
RTSR        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Beginning balance (in shares)   0    
Granted (in shares)   39,000    
Ending balance (in shares)   39,000 0  
Nonvested shares outstanding (in shares)   $ 64.82 $ 0.00  
Granted, weighted average grant date fair value (in dollars per share)   $ 64.82    
Weighted- Average Remaining Term in Years [1]   2 years    
Aggregate Intrinsic Value   $ 3.4    
[1] Midpoint used in calculation.