Debt (Textuals) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Long-term debt	$ 197,000,000	$ 228,600,000
Long-term debt, excluding current portion	197,000,000	226,200,000
Interest expense	10,500,000	11,700,000	$ 15,600,000
Aggregate annual maturities of long-term debt [Abstract]
2019	100,000
2020	29,600,000
2022	42,000,000
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 2017 [Member]
Line of Credit Facility [Line Items]
Variable rate basis	LIBOR
Term Loan Due 2018 [Member]
Line of Credit Facility [Line Items]
Long-term debt		34,900,000
Long-term debt, current portion	$ 2,400,000
Debt issuance costs, net	$ 100,000
Variable rate basis	LIBOR
Basis spread on variable interest rate	150.00%
Long-term debt, gross	$ 34,900,000
Debt instrument, face amount	$ 42,800,000
Debt instrument, Term	5 years
Debt instrument, maturity date	Jan. 31, 2018
Debt instrument, stated interest rate	0.00%
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	29,600,000	26,400,000
Line of Credit Facility, unused commitment level	267,500,000
Debt issuance costs	$ 1,000,000	$ 1,300,000
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%
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Net, Classification	0.7	0.8
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	$ 2,900,000
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%