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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenue, Net	$ 276,279	$ 273,163	$ 263,139	$ 260,927	$ 254,455	$ 245,428	$ 247,006	$ 234,191	$ 1,073,508	$ 981,080	$ 907,648
Cost of Services and Goods Sold	168,393	167,427	163,503	163,645	160,958	155,341	156,380	149,140
Other operating expenses	19,969	21,140	17,767	18,245	17,629	14,707	18,500	11,145
Income Before Interest Expense and Taxes	87,917	84,596	81,869	79,037	75,868	75,380	72,126	73,906	333,419	297,280	250,754
Interest Expense and (Gain) Loss on Extinguishment of Debt	2,483	2,302	2,263	2,269	2,292	2,160	2,178	2,190
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	85,434	82,294	79,606	76,768	73,576	73,220	69,948	71,716	324,102	288,460	238,842
Income Tax Expense (Benefit)	20,718	20,490	19,664	20,036	18,394	18,671	16,997	17,786	80,908	71,848	54,198
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	64,716	61,804	59,942	56,732	55,182	54,549	52,951	53,930	243,194	216,612	184,644
Net Income (Loss) Attributable to Noncontrolling Interest	(8)	(91)	266	216	381	146	35	(21)	383	541	(40)
Net Income (Loss) Attributable to Parent	$ 64,724	$ 61,895	$ 59,676	$ 56,516	$ 54,801	$ 54,403	$ 52,916	$ 53,951	$ 242,811	$ 216,071	$ 184,684
Earnings Per Share, Basic	$ 1.43	$ 1.36	$ 1.30	$ 1.22	$ 1.18	$ 1.15	$ 1.11	$ 1.13	$ 5.31	$ 4.58	$ 3.99
Earnings Per Share, Diluted	$ 1.42	$ 1.35	$ 1.29	$ 1.22	$ 1.17	$ 1.14	$ 1.11	$ 1.13	$ 5.28	$ 4.54	$ 3.82
Weighted Average Number of Shares Outstanding, Basic	45,212	45,526	45,841	46,201	46,541	47,232	47,473	47,606	45,692	47,211	46,286
Weighted Average Number of Shares Outstanding, Diluted	45,517	45,828	46,128	46,493	46,857	47,528	47,791	47,945	45,994	47,553	48,393