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Stock-Based Incentive Plans (Details)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Apr. 20, 2016
$ / shares
shares
Feb. 01, 2016
$ / shares
shares
Sep. 30, 2016
USD ($)
plan
shares
Sep. 30, 2015
USD ($)
Dec. 31, 2015
shares
Stock-Based Compensation          
Number of stock-based incentive plans | plan     1    
Number of stock option plans superseded | plan     2    
Granted (in shares)     0   0
Stock options outstanding and expected to vest     0    
Total intrinsic value of options exercised (in dollars) | $     $ 2.4 $ 1.6  
Unrecognized compensation cost and vesting period          
Unrecognized compensation costs related to unvested share-based compensation arrangements | $     $ 25.4    
Weighted-average period for recognition of unrecognized stock-based compensation expense     1 year 10 months 24 days    
Restricted Stock Units          
Stock-Based Compensation          
Awarded (in shares)   431,439 442,000    
Weighted average granted date fair value (in dollars per share) | $ / shares $ 38.00 $ 32.63      
Total intrinsic value of awards vested (in dollars) | $     $ 10.8 $ 10.1  
Non Employee Directors | Restricted Stock Units          
Stock-Based Compensation          
Awarded (in shares) 1,800