Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Consolidated Statements Of Income [Abstract]
Service revenues and sales	$ 532,256	$ 502,199	$ 1,059,616	$ 1,017,997
Cost of services provided and goods sold (excluding depreciation)	350,493	352,163	690,966	703,908
Selling, general and administrative expenses	93,838	84,513	185,437	155,096
Depreciation	13,612	11,659	25,327	23,047
Amortization	2,510	2,488	5,020	4,965
Other operating expenses/(income)	104	(41,384)	726	(41,142)
Total costs and expenses	460,557	409,439	907,476	845,874
Income from operations	71,699	92,760	152,140	172,123
Interest expense	(379)	(651)	(760)	(1,626)
Other income/(expense) - net	3,785	7,514	7,387	(1,952)
Income before income taxes	75,105	99,623	158,767	168,545
Income taxes	(18,583)	(17,522)	(36,845)	(30,553)
Net income	$ 56,522	$ 82,101	$ 121,922	$ 137,992
Earnings Per Share:
Net income	$ 3.57	$ 5.16	$ 7.66	$ 8.65
Average number of shares outstanding	15,829	15,914	15,919	15,953
Diluted Earnings Per Share:
Net income	$ 3.51	$ 5.01	$ 7.52	$ 8.39
Average number of shares outstanding	16,101	16,373	16,205	16,445
Cash Dividends Per Share	$ 0.34	$ 0.32	$ 0.68	$ 0.64