Lease Obligations (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Rent expense	$ 14,899	$ 13,232	$ 13,960
Rock Hill Facility [Member]
Lease expiration date	Aug. 31, 2021
Number of lease renewal terms | item	2
Lease renewal terms	5 years
Payment of base rent in 2018 through 2020	$ 709
Payment of base rent in 2021	$ 723
Implicit interest rate	6.93%	6.93%
Other Capital Lease Obligations [Member]
Lease expiration date	Aug. 01, 2018
Minimum [Member] | Other Capital Lease Obligations [Member]
Implicit interest rate	1.75%
Maximum [Member] | Other Capital Lease Obligations [Member]
Implicit interest rate		8.06%