Quarterly Information (Schedule Of Quarterly Information) (Details) (USD $) In Thousands, except Per Share data, 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
Quarterly Information [Abstract]
Revenue	$ 120,141	$ 105,563	$ 95,737	$ 100,869	$ 111,674	$ 99,470	$ 88,339	$ 95,896	$ 422,310	$ 395,379	$ 384,952
Gross profit	55,852	56,175	34,712	55,190	49,386	49,086	28,835	52,743
Net income	$ 14,109	$ 15,351	$ 7,584	$ 16,247	$ 15,603	$ 11,328	$ 3,799	$ 13,748	$ 53,291	$ 44,478	$ 42,160
Basic	$ 0.52	$ 0.56	$ 0.28	$ 0.60	$ 0.58	$ 0.42	$ 0.14	$ 0.52	$ 1.96	$ 1.66	$ 1.59
Diluted	$ 0.51	$ 0.55	$ 0.27	$ 0.59	$ 0.57	$ 0.42	$ 0.14	$ 0.51	$ 1.93	$ 1.64	$ 1.58
Dividends per common share	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.40	$ 0.40	$ 0.40