Unaudited Selected Quarterly Results of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenue, Net	$ 243,786	$ 231,344	$ 225,785	$ 206,733	$ 208,193	$ 199,221	$ 198,902	$ 188,337	$ 907,648	$ 794,653	$ 695,539
Cost of Services and Goods Sold	154,034	150,312	152,273	136,750	133,513	125,772	128,917	125,588
Other operating expenses	16,864	17,468	15,709	13,484	13,104	13,232	16,265	11,336
Income Before Interest Expense and Taxes	72,888	63,564	57,803	56,499	61,576	60,217	53,720	51,413	250,754	226,926	186,782
Interest Expense and (Gain) Loss on Extinguishment of Debt	2,358	3,856	2,709	2,989	4,915	4,690	4,114	4,059
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	70,530	59,708	55,094	53,510	56,661	55,527	49,606	47,354	238,842	209,148	171,259
Income Tax Expense (Benefit)	17,371	14,599	14,710	7,518	16,671	16,764	15,244	15,068	54,198	63,747	57,164
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	53,159	45,109	40,384	45,992	39,990	38,763	34,362	32,286	184,644	145,401	114,095
Net Income (Loss) Attributable to Noncontrolling Interest	83	242	(67)	(298)	48	209	146	81	(40)	484	491
Net Income (Loss) Attributable to Parent	$ 53,076	$ 44,867	$ 40,451	$ 46,290	$ 39,942	$ 38,554	$ 34,216	$ 32,205	$ 184,684	$ 144,917	$ 113,604
Earnings Per Share, Basic	$ 1.12	$ 0.96	$ 0.88	$ 1.02	$ 0.89	$ 0.86	$ 0.77	$ 0.72	$ 3.99	$ 3.23	$ 2.47
Earnings Per Share, Diluted	$ 1.11	$ 0.93	$ 0.83	$ 0.94	$ 0.81	$ 0.79	$ 0.71	$ 0.69	$ 3.82	$ 3.00	$ 2.43
Weighted Average Number of Shares Outstanding, Basic	47,343	46,606	45,945	45,225	45,093	44,736	44,651	44,836	46,286	44,830	45,939
Weighted Average Number of Shares Outstanding, Diluted	47,677	48,030	48,662	49,141	49,195	48,955	47,957	46,820	48,393	48,241	46,657