Basis of Presentation and Significant Accounting Policies - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Revenues	$ 504,520		$ 471,231		$ 1,046,728	$ 887,685
Cost of operations	(367,534)		(338,023)		(759,977)	(641,658)
Income before Provision for Income Taxes	84,080		77,734		182,528	140,611
Provision for Income Taxes	(19,684)		(18,734)		(42,512)	(32,501)
Other – net	9,450		6,604		17,367	14,125
Net Income	64,396	$ 75,620	59,000	$ 49,110	140,016	108,110
Accumulated Other Comprehensive Income (Loss) Component Recognized
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	(520)		(696)		(1,550)	(1,066)
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	551		64		550	52
Cost of operations	(430)		(337)		(987)	(135)
Income before Provision for Income Taxes	121		(273)		(437)	(83)
Provision for Income Taxes	(32)		70		106	21
Net Income	89		(203)		(331)	(62)
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	167		155		334	291
Other – net	(776)		(648)		(1,553)	(1,295)
Net Income	$ (609)		$ (493)		$ (1,219)	$ (1,004)