Basis of Presentation and Significant Accounting Policies - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Revenues	$ 519,878			$ 506,000			$ 1,566,606	$ 1,393,685
Cost of operations	(373,783)			(357,732)			(1,133,760)	(999,390)
Income before Provision for Income Taxes	91,882			94,506			274,410	235,117
Provision for Income Taxes	(18,687)			(19,508)			(61,199)	(52,009)
Other – net	10,652			4,670			28,019	18,795
Net Income	73,195	$ 64,396	$ 75,620	74,998	$ 59,000	$ 49,110	213,211	183,108
Accumulated Other Comprehensive Income (Loss) Component Recognized
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	(456)			(447)			(2,006)	(1,513)
Accumulated Other Comprehensive Income (Loss) Component Recognized | Realized gain (loss) on derivative financial instruments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Revenues	(971)			(130)			(421)	(78)
Cost of operations	1,282			228			295	93
Income before Provision for Income Taxes	311			98			(126)	15
Provision for Income Taxes	(79)			(26)			27	(5)
Net Income	232			72			(99)	10
Accumulated Other Comprehensive Income (Loss) Component Recognized | Amortization of prior service cost on benefit obligations
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for Income Taxes	141			130			475	421
Other – net	(829)			(649)			(2,382)	(1,944)
Net Income	$ (688)			$ (519)			$ (1,907)	$ (1,523)