Business Segment Information (Details 2) (USD $) In Thousands, 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
Reconciliation of segment profit to income before income taxes
Segment profit									$ 267,364	$ 270,422	$ 291,145
Stock compensation expense	(3,848)	(4,468)	(4,365)	(5,102)	(4,010)	(4,425)	(4,205)	(4,778)	(17,783)	(17,418)	(15,102)
Depreciation and amortization	(15,316)	(15,239)	(15,152)	(14,781)	(15,335)	(15,069)	(14,267)	(13,952)	(60,488)	(58,623)	(54,682)
Interest expense	(534)	(537)	(576)	(600)	(1,080)	(457)	(494)	(471)	(2,247)	(2,502)	(2,233)
Interest income	400	350	857	412	516	592	858	815	2,019	2,781	3,275
Income before income taxes	$ 58,420	$ 49,537	$ 45,584	$ 35,324	$ 43,309	$ 36,184	$ 57,806	$ 57,361	$ 188,865	$ 194,660	$ 222,403