Segment Reporting Schedule of Revenues from External Customers and Long-Lived Assets (Details) (USD $)	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
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue, Net	$ 254,455,000	$ 245,428,000	$ 247,006,000	$ 234,191,000	$ 243,786,000	$ 231,344,000	$ 225,785,000	$ 206,733,000	$ 981,080,000	$ 907,648,000	$ 794,653,000
Property, Plant and Equipment, Net	125,418,000				115,295,000				125,418,000	115,295,000	104,223,000
Income Before Interest Expense and Taxes	75,868,000	75,380,000	72,126,000	73,906,000	72,888,000	63,564,000	57,803,000	56,499,000	297,280,000	250,754,000	226,926,000
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue, Net									505,200,000	470,600,000	406,823,000
Property, Plant and Equipment, Net	66,092,000				53,566,000				66,092,000	53,566,000	50,160,000
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue, Net									101,387,000	85,287,000	72,296,000
Property, Plant and Equipment, Net	17,263,000				20,113,000				17,263,000	20,113,000	14,396,000
Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue, Net									374,493,000	351,761,000	315,534,000
Property, Plant and Equipment, Net	$ 42,063,000				$ 41,616,000				$ 42,063,000	$ 41,616,000	$ 39,667,000