Fair Value Measurements (Adjustments Reclassified From Accumulated Balances Other Comprehensive Income (Loss) To Earnings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments Gain Loss [Line Items]
Costs of contract revenues	$ 652,620	$ 681,197	$ 760,955
Income tax benefit	35,610	5,792	2,497
Net income (loss)	31,260	8,177	$ 6,189
Fuel Hedge Contracts [Member] | Accumulated Net Gain Loss From Designated Or Qualifying Cash Flow Hedges | Reclassification Out Of Accumulated Other Comprehensive Income
Derivative Instruments Gain Loss [Line Items]
Costs of contract revenues	(358)	50
Income tax benefit	140	20
Net income (loss)	$ (218)	$ 30