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Debt and Credit Arrangements - Senior Secured Revolving Credit Facility (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Aug. 01, 2018
Debt Instrument            
Financing costs amortization $ 300,000 $ 300,000 $ 1,000,000 $ 1,000,000    
Senior Secured Revolving Credit Facility | Revolving Credit Facility            
Debt Instrument            
Debt instrument, term         5 years  
Maximum borrowing capacity $ 450,000,000.0   $ 450,000,000.0      
Debt instrument stated interest rate (percent) 1.00%   1.00%      
Line of credit fronting fee (as a percentage) 0.125%   0.125%      
Maximum percentage of capital stock guaranteed by company 65.00%   65.00%      
Senior Secured Revolving Credit Facility | Revolving Credit Facility | Adjusted Base Rate            
Debt Instrument            
Debt instrument variable interest rate ( percent)     1.00%      
Senior secured revolving credit facility due November 2022 | Swing line loan            
Debt Instrument            
Interest expense   300,000 $ 8,000,000 300,000    
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility            
Debt Instrument            
Line of credit fronting fee (as a percentage) 0.125%   0.125%      
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility            
Debt Instrument            
Debt instrument, term     5 years      
Debt instrument stated interest rate (percent) 4.25%   4.25%      
Debt issuance costs $ 2,500,000   $ 2,500,000      
Unamortized debt issuance costs 2,000,000   2,000,000      
Financing costs amortization 100,000 100,000 400,000 400,000    
Long term debt 298,500,000   298,500,000   $ 239,000,000  
Line of credit outstanding 31,600,000   31,600,000      
Line of credit remaining borrowing amount 119,900,000   $ 119,900,000      
Interest expense 2,900,000          
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility | Minimum            
Debt Instrument            
Line of credit commitment fee (as a percentage)     0.20%      
Line of credit participation fee (as a percentage)     1.50%      
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility | Maximum            
Debt Instrument            
Line of credit commitment fee (as a percentage)     0.375%      
Line of credit participation fee (as a percentage)     2.50%      
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility Sub-limit - Swingline            
Debt Instrument            
Maximum borrowing capacity 25,000,000.0   $ 25,000,000.0      
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility Sub-limit - Letters of Credit            
Debt Instrument            
Maximum borrowing capacity 100,000,000.0   100,000,000.0      
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility Sub-limit - Foreign Currency            
Debt Instrument            
Maximum borrowing capacity 100,000,000.0   100,000,000.0      
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility Sub-limit - Foreign Borrower            
Debt Instrument            
Maximum borrowing capacity 100,000,000.0   100,000,000.0      
Senior secured revolving credit facility due November 2022 | Revolving Credit Facility Sub-limit - Expansion Option            
Debt Instrument            
Maximum borrowing capacity $ 225,000,000.0   $ 225,000,000.0      
Convertible Notes, Due 2018 | Convertible Debt            
Debt Instrument            
Debt instrument stated interest rate (percent)           2.00%
Leverage ratio 3.25   3.25      
Unamortized debt issuance costs $ 0   $ 0   $ 100,000  
Financing costs amortization 0 200,000 100,000 600,000    
Interest expense $ 500,000 $ 4,700,000 $ 2,900,000 $ 13,800,000    
Convertible Notes, Due 2018 | Convertible Debt | Minimum            
Debt Instrument            
Interest to EBITDA 3.0   3.0      
Convertible Notes, Due 2018 | Convertible Debt | Maximum            
Debt Instrument            
Leverage ratio 3.75   3.75