Commitments (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments [Line Items]
Capital expenditures committed, but not yet incurred	$ 1,151	$ 1,612
Future minimum sublease payments expected to be received	1,187	1,936	$ 577
Lease expenditure	27,734	21,956	$ 20,075
Net carrying capital lease amount	14,976	10,692
Purchase commitment amount purchased	$ 35,900
Minimum [Member]
Commitments [Line Items]
Operating lease fixed term	1 month
Capital lease fixed term	1 month
Purchase commitment quantity | item	150,000
Purchase commitment	$ 11,104
Maximum [Member]
Commitments [Line Items]
Operating lease fixed term	20 years
Capital lease fixed term	4 years
Purchase commitment quantity | item	245,900
Purchase commitment	$ 51,028
Computer and Yard Equipment [Member]
Commitments [Line Items]
Net carrying capital lease amount	$ 14,976	$ 10,692