Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Consolidated Statements Of Income [Abstract]
Service revenues and sales	$ 528,297	$ 480,613	$ 1,546,294	$ 1,416,231
Cost of services provided and goods sold (excluding depreciation)	339,240	328,183	1,043,148	973,771
Selling, general and administrative expenses	88,317	76,836	243,413	222,421
Depreciation	11,714	10,147	34,761	29,744
Amortization	2,511	441	7,476	1,366
Other operating (income)/expenses	12,207	78	(28,935)	9,001
Total costs and expenses	453,989	415,685	1,299,863	1,236,303
Income from operations	74,308	64,928	246,431	179,928
Interest expense	(379)	(1,041)	(2,005)	(3,402)
Other income - net	7,675	3,036	5,723	5,488
Income before income taxes	81,604	66,923	250,149	182,014
Income taxes	(13,882)	(7,976)	(44,435)	(27,671)
Net income	$ 67,722	$ 58,947	$ 205,714	$ 154,343
Earnings Per Share:
Net income	$ 4.25	$ 3.69	$ 12.90	$ 9.68
Average number of shares outstanding	15,940	15,970	15,948	15,952
Diluted Earnings Per Share:
Net income	$ 4.14	$ 3.56	$ 12.53	$ 9.35
Average number of shares outstanding	16,373	16,555	16,419	16,514
Cash Dividends Per Share	$ 0.34	$ 0.32	$ 0.98	$ 0.92