Consolidated Statement Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Consolidated Statement Of Income [Abstract]
Service revenues and sales	$ 392,607	$ 386,226	$ 1,173,405	$ 1,144,799
Cost of services provided and goods sold (excluding depreciation)	281,658	272,089	836,348	811,637
Selling, general and administrative expenses	59,373	55,788	181,046	173,267
Depreciation	8,614	8,075	25,619	24,189
Amortization	91	146	274	407
Other operating expenses			4,491
Total costs and expenses	349,736	336,098	1,047,778	1,009,500
Income from operations	42,871	50,128	125,627	135,299
Interest expense	(1,018)	(908)	(2,831)	(2,846)
Other income/(expense) - net	1,640	(2,355)	1,933	(1,256)
Income before income taxes	43,493	46,865	124,729	131,197
Income taxes	(16,664)	(18,032)	(48,175)	(50,852)
Net income	$ 26,829	$ 28,833	$ 76,554	$ 80,345
Earnings Per Share
Net income	$ 1.66	$ 1.71	$ 4.66	$ 4.76
Average number of shares outstanding	16,166	16,865	16,443	16,887
Diluted Earnings Per Share
Net income	$ 1.62	$ 1.65	$ 4.54	$ 4.61
Average number of shares outstanding	16,559	17,422	16,851	17,430
Cash Dividends Per Share	$ 0.26	$ 0.24	$ 0.74	$ 0.68