Segment Reporting Schedule of Revenues from External Customers and Long-Lived Assets (Details) (USD $)	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
Revenues from External Customers and Long-Lived Assets [Line Items]
US Revenue from exports									$ 59,800,000	$ 49,700,000	$ 42,800,000
Revenue, Net	243,786,000	231,344,000	225,785,000	206,733,000	208,193,000	199,221,000	198,902,000	188,337,000	907,648,000	794,653,000	695,539,000
Income Before Interest Expense and Taxes	72,888,000	63,564,000	57,803,000	56,499,000	61,576,000	60,217,000	53,720,000	51,413,000	250,754,000	226,926,000	186,782,000
Assets	605,139,000				650,241,000				605,139,000	650,241,000	658,166,000
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue, Net									470,600,000	406,823,000	339,235,000
Income Before Interest Expense and Taxes									157,267,000	126,726,000	112,158,000
Assets	321,756,000				355,269,000				321,756,000	355,269,000	326,223,000
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue, Net									85,287,000	72,296,000	54,888,000
Income Before Interest Expense and Taxes									27,651,000	34,152,000	14,430,000
Assets	73,385,000				69,260,000				73,385,000	69,260,000	45,344,000
Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue, Net									351,761,000	315,534,000	301,416,000
Income Before Interest Expense and Taxes									65,836,000	66,048,000	60,194,000
Assets	$ 209,998,000				$ 225,712,000				$ 209,998,000	$ 225,712,000	$ 286,599,000