Consolidated Statement Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Consolidated Statement Of Income [Abstract]
Service revenues and sales	$ 390,409	$ 381,921	$ 780,798	$ 758,573
Cost of services provided and goods sold (excluding depreciation)	276,255	270,663	554,690	539,548
Selling, general and administrative expenses	62,628	58,442	121,673	117,479
Depreciation	8,581	8,082	17,005	16,114
Amortization	91	134	183	261
Other operating expenses	4,491		4,491
Total costs and expenses	352,046	337,321	698,042	673,402
Income from operations	38,363	44,600	82,756	85,171
Interest expense	(971)	(969)	(1,813)	(1,938)
Other income/(expense) - net	3,217	536	293	1,099
Income before income taxes	40,609	44,167	81,236	84,332
Income taxes	(15,724)	(17,192)	(31,511)	(32,820)
Net income	$ 24,885	$ 26,975	$ 49,725	$ 51,512
Earnings Per Share
Net income	$ 1.51	$ 1.60	$ 3.00	$ 3.05
Average number of shares outstanding	16,443	16,880	16,583	16,872
Diluted Earnings Per Share
Net income	$ 1.48	$ 1.55	$ 2.93	$ 2.96
Average number of shares outstanding	16,831	17,419	16,999	17,419
Cash Dividends Per Share	$ 0.24	$ 0.22	$ 0.48	$ 0.44