Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Consolidated Statements Of Income [Abstract]
Service revenues and sales (Note 2)	$ 2,139,261	$ 2,079,583	$ 1,938,555
Cost of services provided and goods sold (excluding depreciation)	1,369,458	1,378,197	1,321,126
Selling, general and administrative expenses	366,727	330,218	305,712
Depreciation	49,011	46,596	40,870
Amortization	10,040	9,987	4,335
Other operating (income)/expenses (Note 19)	987	(75,095)	9,132
Total costs and expenses	1,796,223	1,689,903	1,681,175
Income from operations	343,038	389,680	257,380
Interest expense	(1,868)	(2,355)	(4,535)
Other income--net (Note 10)	9,144	8,665	8,764
Income before income taxes	350,314	395,990	261,609
Income taxes (Note 11)	(81,764)	(76,524)	(41,686)
Net income	$ 268,550	$ 319,466	$ 219,923
Earnings Per Share (Note 15)
Net income	$ 17.14	$ 20.02	$ 13.77
Average number of shares outstanding	15,671	15,955	15,969
Diluted Earnings Per Share (Note 15)
Net income	$ 16.85	$ 19.48	$ 13.31
Average number of shares outstanding	15,938	16,398	16,527