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Stock-Based Compensation - Narrative (Details) - USD ($)
$ / shares in Units, shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized (in shares) 2,200    
Number of shares available for grant (in shares) 551    
Allocated stock-based compensation expense $ 10,100,000 $ 5,500,000 $ 2,700,000
Unrecognized compensation cost $ 17,200,000    
Compensation cost not yet recognized, period for recognition 1 year 9 months    
Options, weighted average grant date fair value (in USD per share)   $ 14.28 $ 18.06
Proceeds from exercise of stock options and other $ 700,000 $ 10,000,000 $ 5,100,000
Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration period 10 years    
Award vesting period 4 years    
Performance Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years 3 years  
Weighted average grant date fair value (in USD per share) $ 47.88 $ 29.77 $ 59.50
Fair value of awards vested $ 0 $ 0 $ 600,000
Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value (in USD per share) $ 45.61 $ 32.36 $ 56.13
Fair value of awards vested $ 1,600,000 $ 300,000 $ 600,000
THC      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated stock-based compensation expense   2,000,000 1,800,000
Minimum | Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration period 5 years    
Minimum | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 1 year    
Maximum | Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration period 10 years    
Maximum | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Non-HERC Employee | Nonemployee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Proceeds from exercise of stock options and other   $ 9,600,000 $ 5,100,000