Business Segment Information (Details 2) (USD $) In Thousands, 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
Reconciliation of segment profit to income before income taxes
Segment profit									$ 270,422	$ 291,145	$ 227,237
Stock compensation expense	(4,010)	(4,425)	(4,205)	(4,778)	(3,272)	(3,596)	(3,706)	(4,528)	(17,418)	(15,102)	(19,782)
Depreciation and amortization	(15,335)	(15,069)	(14,267)	(13,952)	(13,075)	(13,950)	(14,235)	(13,422)	(58,623)	(54,682)	(47,268)
Interest expense	(1,080)	(457)	(494)	(471)	(482)	(482)	(584)	(685)	(2,502)	(2,233)	(2,424)
Interest income	516	592	858	815	809	846	803	817	2,781	3,275	6,245
Income before income taxes	$ 43,309	$ 36,184	$ 57,806	$ 57,361	$ 45,172	$ 72,921	$ 60,776	$ 43,534	$ 194,660	$ 222,403	$ 164,008