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Debt (Textuals) (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]      
Long-term debt $ 196,100,000 $ 197,000,000  
Long-term debt, excluding current portion 196,000,000 197,000,000  
Interest expense 9,300,000 10,500,000 $ 11,700,000
Aggregate annual maturities of long-term debt [Abstract]      
2019 100,000    
2020 28,600,000    
2021 0    
2022 42,000,000    
2023 0    
Thereafter $ 126,000,000    
Senior A Notes Due 2022 [Member]      
Line of Credit Facility [Line Items]      
Debt instrument, maturity date Jul. 05, 2022    
Debt instrument, stated interest rate 3.67%    
Senior B Notes Due 2024 [Member]      
Line of Credit Facility [Line Items]      
Debt instrument, maturity date Jul. 05, 2024    
Debt instrument, stated interest rate 3.82%    
Senior C Notes Due 2027 [Member]      
Line of Credit Facility [Line Items]      
Debt instrument, maturity date Jul. 05, 2027    
Debt instrument, stated interest rate 4.02%    
Revolving credit facility, due 2020 [Member]      
Line of Credit Facility [Line Items]      
Line of Credit Facility, current borrowing capacity $ 300,000,000.0    
Line of Credit Facility, additional borrowing capacity 100,000,000    
Credit Facility is available for swing-line loans 30,000,000.0    
Credit available for the issuance of letters of credit 30,000,000.0    
Line of credit, noncurrent 28,600,000 29,600,000  
Line of Credit Facility, unused commitment level $ 268,900,000    
Variable rate basis LIBOR    
Debt issuance costs $ 600,000 $ 1,000,000  
Debt instrument, maturity date Oct. 31, 2020    
Debt instrument, stated interest rate 1.00%    
Senior Notes [Member]      
Line of Credit Facility [Line Items]      
Debt instrument, face amount $ 168,000,000    
Weighted average of the coupon interest rate 3.87%    
Debt issuance costs 0.6 0.7  
Senior Notes [Member] | Senior A Notes Due 2022 [Member]      
Line of Credit Facility [Line Items]      
Long-term debt $ 42,000,000 $ 42,000,000  
Debt instrument, face amount $ 42,000,000    
Debt instrument, stated interest rate 3.67%    
Senior Notes [Member] | Senior B Notes Due 2024 [Member]      
Line of Credit Facility [Line Items]      
Long-term debt $ 53,000,000 53,000,000  
Debt instrument, face amount $ 53,000,000    
Debt instrument, stated interest rate 3.82%    
Senior Notes [Member] | Senior C Notes Due 2027 [Member]      
Line of Credit Facility [Line Items]      
Long-term debt $ 73,000,000 $ 73,000,000  
Debt instrument, face amount $ 73,000,000    
Debt instrument, stated interest rate 4.02%    
Letter of Credit [Member] | Revolving credit facility, due 2020 [Member]      
Line of Credit Facility [Line Items]      
Outstanding amount $ 2,500,000    
Minimum [Member] | Base Rate [Member] | Revolving credit facility, due 2020 [Member]      
Line of Credit Facility [Line Items]      
Basis spread on variable interest rate 0.00%    
Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member] | Revolving credit facility, due 2020 [Member]      
Line of Credit Facility [Line Items]      
Basis spread on variable interest rate 100.00%    
Maximum [Member] | Base Rate [Member] | Revolving credit facility, due 2020 [Member]      
Line of Credit Facility [Line Items]      
Basis spread on variable interest rate 75.00%    
Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member] | Revolving credit facility, due 2020 [Member]      
Line of Credit Facility [Line Items]      
Basis spread on variable interest rate 175.00%