Fair Value Measurements (Adjustments Reclassified from Accumulated Balances Other Comprehensive Income (Loss) to Earnings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments Gain Loss [Line Items]
Other expense	$ (2,590)	$ 11	$ (777)
Costs of contract revenues	509,335	549,429	552,130
Income tax (provision) benefit	(5,437)	33,761	177
Net income (loss)	6,293	31,260	8,177
Accumulated Foreign Currency Adjustment Attributable to Parent [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member]
Derivative Instruments Gain Loss [Line Items]
Other expense	(2,337)
Income tax (provision) benefit	612
Net income (loss)	(1,725)
Fuel Hedge Contracts [Member] | Accumulated Gain Loss Net Cash Flow Hedge Parent [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member]
Derivative Instruments Gain Loss [Line Items]
Costs of contract revenues	(2,125)	(358)	50
Income tax (provision) benefit	(556)	140	20
Net income (loss)	$ (1,569)	$ (218)	$ 30