Credit Arrangements - Background (Details) $ in Thousands, CAD in Millions				12 Months Ended
	Nov. 09, 2015 USD ($) payment	Jun. 03, 2015 USD ($)	Jul. 25, 2013 USD ($) payment	Dec. 31, 2017 USD ($) building	Dec. 31, 2015 USD ($) loan building	Dec. 31, 2017 CAD	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 29, 2017 USD ($)	Sep. 28, 2017 USD ($)	Dec. 28, 2012 USD ($)
Senior Notes
Credit arrangements
Initial principal amount	$ 25,000										$ 50,000
Interest rate (as a percent)	4.60%					3.65%	3.65%
Required principal payment	$ 3,600			$ 7,100
Number of annual principal payments | payment	7
Senior Notes | Minimum
Credit arrangements
Prepayment to be paid on debt				$ 5,000
Notes Agreement
Credit arrangements
Initial principal amount			$ 25,000								$ 25,000
Interest rate (as a percent)			3.85%
Required principal payment			$ 3,600
Number of annual principal payments | payment			7
Notes Agreement | Minimum
Credit arrangements
Restrictions on investments, change of control provisions and provisions as a percentage of total assets to be disposed off				20.00%
Notes Agreement | Maximum
Credit arrangements
Initial principal amount		$ 75,000
Mortgages
Credit arrangements
Number of assets secured | building				3
Interest rate (as a percent)						5.00%	5.00%
Assumed notes							$ 4,200
Commercial equipment notes | Minimum
Credit arrangements
Interest rate (as a percent)						1.78%	1.78%
Commercial equipment notes | Maximum
Credit arrangements
Interest rate (as a percent)						3.51%	3.51%
Secured mortgage notes, maturing on January 1, 2031
Credit arrangements
Number of secured mortgage notes payable to a bank | loan					2
Initial principal amount					$ 8,000
Number of assets secured | building					2
Interest rate (as a percent)					4.30%
Credit Agreement
Credit arrangements
Maximum borrowing capacity									$ 200,000	$ 125,000
Debt issuance costs								$ 600
Available borrowing capacity							$ 180,500
Additional period of time to issue notes		3 years
Credit Agreement | Federal funds rate
Credit arrangements
Basis spread on variable rate (as a percent)				0.50%
Credit Agreement | Minimum
Credit arrangements
Prepayment to be paid on debt				$ 5,000
Restrictions on investments, change of control provisions and provisions as a percentage of total assets to be disposed off				20.00%
Credit Agreement | Revolving credit facility
Credit arrangements
Maximum borrowing capacity							200,000
Borrowings outstanding							0
Credit Agreement | Commercial letters of credit
Credit arrangements
Total commercial letters of credit outstanding							19,500
Canadian Credit Facility
Credit arrangements
Available borrowing capacity | CAD						CAD 7.5
Canadian Credit Facility | Commercial letters of credit
Credit arrangements
Maximum borrowing capacity | CAD						CAD 8.0
Total commercial letters of credit outstanding							$ 500
Annual fee (as a percent)				1.00%
Canadian Credit Facility | Commercial letters of credit | Maximum
Credit arrangements
Term of credit facility				5 years