Fair Value Measurements (Adjustments Reclassified from Accumulated Balances Other Comprehensive Income (Loss) to Earnings) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments Gain Loss [Line Items]
Other expense	$ 123	$ 33	$ 295	$ (2,032)
Costs of contract revenues	147,325	113,138	290,085	232,631
Income tax (provision) benefit	(4,230)	(818)	(11,076)	1,657
Net income (loss)	(8,246)	974	(25,414)	10,295
Accumulated Foreign Currency Adjustment Attributable to Parent [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member]
Derivative Instruments Gain Loss [Line Items]
Other expense				(2,015)
Income tax (provision) benefit				527
Net income (loss)				(1,487)
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	399	(1,404)	1,606	(2,396)
Income tax (provision) benefit	104	(367)	420	(627)
Net income (loss)	$ 295	$ (1,037)	$ 1,186	$ (1,769)