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Debt and Credit Arrangements - Senior Secured Revolving Credit Facility (Details) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Aug. 03, 2011
Debt Instrument            
Financing costs amortization $ 400,000 $ 400,000 $ 700,000 $ 700,000    
Short-term debt outstanding 66,800,000   66,800,000   $ 58,900,000  
Revolving Credit Facility            
Debt Instrument            
Long term debt 244,000,000   244,000,000   $ 239,000,000  
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      
Line of credit fronting fee (as a percentage) 0.125%   0.125%      
Debt instrument stated interest rate 1.00%   1.00%      
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            
Variable rate (as a percentage)     1.00%      
Senior secured revolving credit facility due November 2022 | Swing line loan            
Debt Instrument            
Debt instrument stated interest rate 6.25%   6.25%      
Short-term debt outstanding $ 6,000,000   $ 6,000,000      
Interest expense $ 5,100,000   $ 5,100,000      
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 issuance costs $ 2,500,000   $ 2,500,000      
Unamortized debt issuance costs 2,200,000   2,200,000      
Financing costs amortization 200,000 200,000 300,000 300,000    
Long term debt $ 238,000,000   $ 238,000,000      
Debt instrument stated interest rate 4.25%   4.25%      
Interest expense $ 2,600,000   $ 2,600,000      
Line of credit outstanding 48,800,000   48,800,000      
Line of credit remaining borrowing amount 157,200,000   $ 157,200,000      
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            
Leverage ratio 3.25   3.25      
Unamortized debt issuance costs $ 0   $ 0   $ 100,000  
Financing costs amortization 100,000 200,000 100,000 400,000    
Debt instrument stated interest rate           2.00%
Interest expense $ 1,000,000 $ 4,500,000 $ 2,400,000 $ 9,100,000    
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