Fair Value Measurements (Adjustments Reclassified from Accumulated Balances Other Comprehensive Income (Loss) to Earnings) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments Gain Loss [Line Items]
Other expense	$ (1,121)	$ 172
Costs of contract revenues	149,221	142,760
Income tax benefit	(11,310)	(6,846)
Net income (loss)	(33,987)	(17,168)
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	1,792	1,207
Income tax benefit	454	316
Net income (loss)	$ 1,338	$ 891