Commitments (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments [Line Items]
Capital expenditures committed, but not yet incurred	$ 1,612	$ 3,197
Future minimum sublease payments expected to be received	1,936	577	$ 1,077
Lease expenditure	21,956	20,075	$ 17,367
Net carrying capital lease amount	$ 10,692	3,968
Minimum [Member]
Commitments [Line Items]
Operating lease fixed term	1 month
Capital lease fixed term	1 month
Maximum [Member]
Commitments [Line Items]
Operating lease fixed term	20 years
Capital lease fixed term	4 years
Computer and Yard Equipment [Member]
Commitments [Line Items]
Net carrying capital lease amount	$ 10,692	$ 3,968