Financial Instruments (Schedule of Long-Term Debt) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt	$ 485,781	$ 265,096
Less: current portion	(566)	(16)
Long-term debt, net of current portion	485,215	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	$ 412,000	$ 215,000
Interest rate at end of period	2.53%	2.27%
Various Notes and Mortgages [Member]
Debt Instrument [Line Items]
Long-term debt	$ 88	$ 96
Interest rate at end of period	5.50%	5.50%
Maturity date range, end	Dec. 31, 2021
Obligation under Capital Lease [Member]
Debt Instrument [Line Items]
Long-term debt	$ 23,693
Maturity date range, end	Dec. 31, 2027