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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenue, Net	$ 254,455	$ 245,428	$ 247,006	$ 234,191	$ 243,786	$ 231,344	$ 225,785	$ 206,733	$ 981,080	$ 907,648	$ 794,653
Cost of Services and Goods Sold	160,958	155,341	156,380	149,140	154,034	150,312	152,273	136,750
Other operating expenses	17,629	14,707	18,500	11,145	16,864	17,468	15,709	13,484
Income Before Interest Expense and Taxes	75,868	75,380	72,126	73,906	72,888	63,564	57,803	56,499	297,280	250,754	226,926
Interest Expense and (Gain) Loss on Extinguishment of Debt	2,292	2,160	2,178	2,190	2,358	3,856	2,709	2,989
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	73,576	73,220	69,948	71,716	70,530	59,708	55,094	53,510	288,460	238,842	209,148
Income Tax Expense (Benefit)	18,394	18,671	16,997	17,786	17,371	14,599	14,710	7,518	71,848	54,198	63,747
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	55,182	54,549	52,951	53,930	53,159	45,109	40,384	45,992	216,612	184,644	145,401
Net Income (Loss) Attributable to Noncontrolling Interest	381	146	35	(21)	83	242	(67)	(298)	541	(40)	484
Net Income (Loss) Attributable to Parent	$ 54,801	$ 54,403	$ 52,916	$ 53,951	$ 53,076	$ 44,867	$ 40,451	$ 46,290	$ 216,071	$ 184,684	$ 144,917
Earnings Per Share, Basic	$ 1.18	$ 1.15	$ 1.11	$ 1.13	$ 1.12	$ 0.96	$ 0.88	$ 1.02	$ 4.58	$ 3.99	$ 3.23
Earnings Per Share, Diluted	$ 1.17	$ 1.14	$ 1.11	$ 1.13	$ 1.11	$ 0.93	$ 0.83	$ 0.94	$ 4.54	$ 3.82	$ 3.00
Weighted Average Number of Shares Outstanding, Basic	46,541	47,232	47,473	47,606	47,343	46,606	45,945	45,225	47,211	46,286	44,830
Weighted Average Number of Shares Outstanding, Diluted	46,857	47,528	47,791	47,945	47,677	48,030	48,662	49,141	47,553	48,393	48,241