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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Derivative Instruments Gain Loss [Line Items]
Costs of contract revenues	$ 146,992	$ 134,904	$ 291,549	$ 284,125
Income tax (provision) benefit	(829)	(3,131)	(2,218)	(14,441)
Net income (loss)	(2,105)	(8,960)	(10,919)	(42,947)
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	(2,513)	3,340	(4,554)	5,132
Income tax (provision) benefit	(635)	850	(1,150)	1,304
Net income (loss)	$ (1,878)	$ 2,490	$ (3,404)	$ 3,828