Financial Commitments (Long-Term Debt) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total debt	$ 822,614	$ 736,276
Less: Current maturities	28,105	30,748
Long-term debt, net	794,509	705,528
Convertible Notes [Member]
Debt Instrument [Line Items]
Total debt	166,442	161,635
Equipment Financing And Mortgages [Member]
Debt Instrument [Line Items]
Total debt	64,434	76,820
Other Indebtedness [Member]
Debt Instrument [Line Items]
Total debt	6,617	5,087
2017 Senior Notes [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Total debt	493,121	$ 492,734
2017 Credit Facility [Member] | Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	$ 92,000