Financial Instruments (Schedule of Long-Term Debt) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 491,041	$ 524,931
Less: current portion	(19)	(1,224)
Long-term debt, net of current portion	491,022	523,707
Credit Agreement [Member]
Debt Instrument [Line Items]
Long-term debt	$ 491,000	$ 499,000
Interest rate	3.61%	3.69%	3.69%
Maturity date range, end	Dec. 31, 2022
Finance lease obligation [Member]
Debt Instrument [Line Items]
Long-term debt		$ 25,886
Other Debt [Member]
Debt Instrument [Line Items]
Long-term debt	$ 41	$ 45
Interest rate	5.50%
Maturity date range, end	Dec. 31, 2021