Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Consolidated Statements Of Income [Abstract]
Service revenues and sales	$ 480,613	$ 444,151	$ 1,416,231	$ 1,325,140
Cost of services provided and goods sold (excluding depreciation)	328,183	305,312	973,771	915,589
Selling, general and administrative expenses	76,836	67,177	222,421	204,474
Depreciation	10,147	9,657	29,744	28,642
Amortization	441	35	1,366	96
Other operating expenses	78	257	9,001	88
Total costs and expenses	415,685	382,438	1,236,303	1,148,889
Income from operations	64,928	61,713	179,928	176,251
Interest expense	(1,041)	(1,082)	(3,402)	(3,813)
Other income - net	3,036	2,300	5,488	4,356
Income before income taxes	66,923	62,931	182,014	176,794
Income taxes	(7,976)	(11,682)	(27,671)	(25,578)
Net income	$ 58,947	$ 51,249	$ 154,343	$ 151,216
Earnings Per Share:
Net income	$ 3.69	$ 3.19	$ 9.68	$ 9.41
Average number of shares outstanding	15,970	16,074	15,952	16,070
Diluted Earnings Per Share:
Net income	$ 3.56	$ 3.06	$ 9.35	$ 8.98
Average number of shares outstanding	16,555	16,772	16,514	16,830
Cash Dividends Per Share	$ 0.32	$ 0.30	$ 0.92	$ 0.86