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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue, Net	$ 278,622	$ 276,135	$ 267,562	$ 262,903	$ 276,279	$ 273,163	$ 263,139	$ 260,927	$ 1,085,222	$ 1,073,508	$ 981,080
Cost of Services and Goods Sold	168,517	166,927	168,158	161,669	168,393	167,427	163,503	163,645
Other operating expenses	19,655	20,088	15,270	18,407	19,969	21,140	17,767	18,245
Income Before Interest Expense and Taxes	90,450	89,120	84,134	82,827	87,917	84,596	81,869	79,037	346,531	333,419	297,280
Interest Expense and (Gain) Loss on Extinguishment of Debt	2,882	2,561	2,794	2,363	2,483	2,302	2,263	2,269
Income before income tax expense	87,568	86,559	81,340	80,464	85,434	82,294	79,606	76,768	335,931	324,102	288,460
Income Tax Expense (Benefit)	20,841	19,909	17,244	19,311	20,718	20,490	19,664	20,036	77,305	80,908	71,848
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	66,727	66,650	64,096	61,153	64,716	61,804	59,942	56,732	258,626	243,194	216,612
Net Income (Loss) Attributable to Noncontrolling Interest	537	153	362	89	(8)	(91)	266	216	1,141	383	541
Net Income (Loss) Attributable to Parent	$ 66,190	$ 66,497	$ 63,734	$ 61,064	$ 64,724	$ 61,895	$ 59,676	$ 56,516	$ 257,485	$ 242,811	$ 216,071
Earnings Per Share, Basic	$ 1.51	$ 1.51	$ 1.43	$ 1.36	$ 1.43	$ 1.36	$ 1.30	$ 1.22	$ 5.80	$ 5.31	$ 4.58
Earnings Per Share, Diluted	$ 1.51	$ 1.50	$ 1.42	$ 1.35	$ 1.42	$ 1.35	$ 1.29	$ 1.22	$ 5.77	$ 5.28	$ 4.54
Weighted Average Number of Shares Outstanding, Basic	43,742	44,152	44,660	44,908	45,212	45,526	45,841	46,201	44,362	45,692	47,211
Weighted Average Number of Shares Outstanding, Diluted	43,927	44,381	44,910	45,182	45,517	45,828	46,128	46,493	44,600	45,994	47,553