Credit Arrangements (Details)	3 Months Ended		3 Months Ended	0 Months Ended		3 Months Ended		3 Months Ended					3 Months Ended		3 Months Ended
	Mar. 31, 2015 Senior Notes USD ($)	Dec. 28, 2012 Senior Notes USD ($)	Mar. 31, 2015 Senior Notes Minimum USD ($)	Jul. 25, 2013 Notes Agreement USD ($) payment	Dec. 28, 2012 Notes Agreement USD ($)	Mar. 31, 2015 Notes Agreement Minimum	Mar. 31, 2015 Credit Agreement USD ($)	Mar. 31, 2015 Credit Agreement Federal funds rate	Mar. 31, 2015 Credit Agreement Minimum USD ($)	Mar. 31, 2015 Credit Agreement Revolving line of credit USD ($)	Mar. 31, 2015 Credit Agreement Commercial letters of credit USD ($)	Dec. 31, 2014 Credit Agreement Commercial letters of credit USD ($)	Mar. 31, 2015 Canadian Credit Facility Commercial letters of credit CAD	Dec. 31, 2014 Canadian Credit Facility Commercial letters of credit CAD	Mar. 31, 2015 Canadian Credit Facility Commercial letters of credit Maximum
Credit arrangements
Maximum borrowing capacity							$ 125,000,000			$ 125,000,000			8,000,000
Basis spread on variable rate (as a percent)								0.50%
Prepayment to be paid on debt			5,000,000						5,000,000
Interest rate (as a percent)	3.65%			3.85%
Restrictions on investments, change of control provisions and provisions as a percentage of total assets to be disposed off						20.00%			20.00%
Total commercial letters of credit outstanding											3,620,000	4,659,000	2,505,000	2,563,000
Available borrowing capacity							121,380,000						5,495,000
Borrowings outstanding										0
Term of credit facility															5 years
Annual fee (as a percent)													1.00%
Required principal payment	7,100,000			3,600,000
Initial principal amount		$ 50,000,000		$ 25,000,000	$ 25,000,000
Number of annual principal payments				7