Financial Instruments (Schedule of Long-Term Debt) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt	$ 491,465	$ 265,096
Less: current portion	(1,462)	(16)
Long-term debt, net of current portion	490,003	265,080
Private Placement, Notes [Member]
Debt Instrument [Line Items]
Long-term debt	$ 50,000	$ 50,000
Interest rate	6.84%	6.84%
Maturity date range, end	Dec. 31, 2017
Credit Agreement [Member]
Debt Instrument [Line Items]
Long-term debt	$ 425,000	$ 215,000
Interest rate at end of period	2.53%	2.36%
Various Notes and Mortgages [Member]
Debt Instrument [Line Items]
Long-term debt	$ 84	$ 96
Interest rate at end of period	5.50%	5.50%
Maturity date range, end	Dec. 31, 2021
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Long-term debt	$ 16,381
Maturity date range, end	Dec. 31, 2022