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Basis of Presentation and Significant Accounting Policies - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                      
Revenues $ 556,910 $ 519,878 $ 504,520 $ 542,208 $ 501,235 $ 506,000 $ 471,231 $ 416,454 $ 2,123,516 $ 1,894,920 $ 1,799,889
Cost of operations                 (1,548,119) (1,361,056) (1,295,876)
Total before tax                 362,172 313,742 280,145
Other – net                 34,116 22,599 512
Provision for Income Taxes                 (82,976) (69,065) (52,840)
Net income                 279,196 244,677 227,305
Accumulated Other Comprehensive Income Component Recognized                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                      
Net income                 (2,045) (2,918) (1,380)
Accumulated Other Comprehensive Income Component Recognized | Realized (loss) gain on derivative financial instruments                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                      
Revenues                 (35)    
Cost of operations                 695    
Total before tax                 660    
Provision for Income Taxes                 (173)    
Net income                 487    
Accumulated Other Comprehensive Income Component Recognized | Realized (loss) gain on derivative financial instruments                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                      
Revenues                   (132) (242)
Cost of operations                   (1,066) 407
Total before tax                   (1,198) 165
Provision for Income Taxes                   310 (47)
Net income                   (888) 118
Accumulated Other Comprehensive Income Component Recognized | Amortization of prior service cost on benefit obligations                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                      
Other – net                 (3,177) (2,592) (1,910)
Provision for Income Taxes                 645 562 410
Net income                 (2,532) (2,030) (1,500)
Accumulated Other Comprehensive Income Component Recognized | Realized gains on investments                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                      
Other – net                 0 0 3
Provision for Income Taxes                 0 0 (1)
Net income                 $ 0 $ 0 $ 2