Basis of Presentation and Significant Accounting Policies - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Revenues	$ 457,463	$ 428,229
Cost of operations	(327,364)	(303,216)
Total before tax	85,016	80,378
Other – net	7,910	7,486
Provision for Income Taxes	(18,603)	(24,592)
Net Income	66,413	55,786
Accumulated Other Comprehensive Income (Loss) Component Recognized
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	(243)	(239)
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	(11)	(4)
Cost of operations	120	54
Total before tax	109	50
Provision for Income Taxes	(30)	(13)
Net Income	79	37
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]
Other – net	(505)	(446)
Provision for Income Taxes	183	156
Net Income	(322)	(290)
Accumulated Other Comprehensive Income (Loss) Component Recognized | Realized gain on investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other – net	0	22
Provision for Income Taxes	0	(8)
Net Income	$ 0	$ 14