Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Consolidated Statements Of Income [Abstract]
Service revenues and sales (Note 2)	$ 2,079,583	$ 1,938,555	$ 1,782,648
Cost of services provided and goods sold (excluding depreciation)	1,378,197	1,321,126	1,227,644
Selling, general and administrative expenses	330,218	305,712	271,209
Depreciation	46,596	40,870	38,464
Amortization	9,987	4,335	399
Other operating (income)/expenses (Note 19)	(75,095)	9,132	1,300
Total costs and expenses	1,689,903	1,681,175	1,539,016
Income from operations	389,680	257,380	243,632
Interest expense	(2,355)	(4,535)	(4,990)
Other income--net (Note 10)	8,665	8,764	958
Income before income taxes	395,990	261,609	239,600
Income taxes (Note 11)	(76,524)	(41,686)	(34,056)
Net income	$ 319,466	$ 219,923	$ 205,544
Earnings Per Share (Note 15)
Net income	$ 20.02	$ 13.77	$ 12.80
Average number of shares outstanding	15,955	15,969	16,059
Diluted Earnings Per Share (Note 15)
Net income	$ 19.48	$ 13.31	$ 12.23
Average number of shares outstanding	16,398	16,527	16,803