Unaudited Consolidated Statement Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Unaudited Consolidated Statement Of Income [Abstract]
Service revenues and sales	$ 417,444	$ 392,607	$ 1,238,367	$ 1,173,405
Cost of services provided and goods sold (excluding depreciation)	288,047	281,658	859,039	836,348
Selling, general and administrative expenses	66,919	59,373	205,031	181,046
Depreciation	8,819	8,614	26,545	25,619
Amortization	33	91	111	274
Other operating expenses/(income)	(371)		91,138	4,491
Total costs and expenses	363,447	349,736	1,181,864	1,047,778
Income from operations	53,997	42,871	56,503	125,627
Interest expense	(1,048)	(1,018)	(3,164)	(2,831)
Other income - net	1,323	1,640	5,439	1,933
Income before income taxes	54,272	43,493	58,778	124,729
Income taxes	(18,835)	(16,664)	(15,153)	(48,175)
Net income	$ 35,437	$ 26,829	$ 43,625	$ 76,554
Earnings Per Share
Net income	$ 2.22	$ 1.66	$ 2.72	$ 4.66
Average number of shares outstanding	15,976	16,166	16,068	16,443
Diluted Earnings Per Share
Net income	$ 2.13	$ 1.62	$ 2.60	$ 4.54
Average number of shares outstanding	16,676	16,559	16,763	16,851
Cash Dividends Per Share	$ 0.28	$ 0.26	$ 0.80	$ 0.74