Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Consolidated Statements Of Income [Abstract]
Service revenues and sales	$ 473,584	$ 441,813	$ 935,618	$ 880,989
Cost of services provided and goods sold (excluding depreciation)	323,637	305,741	645,588	610,277
Selling, general and administrative expenses	71,556	68,297	145,585	137,297
Depreciation	9,887	9,718	19,597	18,985
Amortization	406	34	925	61
Other operating expenses/(income)	2,570	(118)	8,923	(169)
Total costs and expenses	408,056	383,672	820,618	766,451
Income from operations	65,528	58,141	115,000	114,538
Interest expense	(1,237)	(1,524)	(2,361)	(2,731)
Other income - net	13	1,038	2,452	2,056
Income before income taxes	64,304	57,655	115,091	113,863
Income taxes	(13,575)	(2,684)	(19,695)	(13,896)
Net income	$ 50,729	$ 54,971	$ 95,396	$ 99,967
Earnings Per Share:
Net income	$ 3.18	$ 3.43	$ 5.98	$ 6.22
Average number of shares outstanding	15,928	16,035	15,941	16,067
Diluted Earnings Per Share:
Net income	$ 3.08	$ 3.27	$ 5.79	$ 5.93
Average number of shares outstanding	16,449	16,811	16,489	16,854
Cash Dividends Per Share	$ 0.30	$ 0.28	$ 0.60	$ 0.56