Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Long-term portion [Abstract]
Floating Rate Senior Notes (described below)		$ 100,000,000
Total long-term portion	230,882,000	247,300,000
Total debt	230,882,000	247,300,000
Unsecured syndicated senior credit facility [Abstract]
Revolving credit facility, maximum borrowing capacity	430,000,000
Revolving credit facility, term (in years)	5
Potential increase in borrowing capacity	75,000,000
Line of Credit Facility, Amount Outstanding	230,882,000
Line of credit facility, remaining borrowing capacity	195,484,000
Weighted average effective interest rate (in hundredths)	2.20%
Line of credit facility, interest rate description	Revolving borrowings under the Credit Facility bear interest, at our option, at either of the following and in each case plus an applicable margin:a.a base rate, which is the highest of (i) the Wells Fargo Bank, National Association prime rate, (ii) the Federal Funds Rate plus 0.500% and (iii) the LIBOR Market Index Rate plus 1.000%; or b.the London Interbank Offered Rate (LIBOR). Borrowings by the Canadian Borrower bear interest, at the Canadian Borrower’s option, at either of the following and in each case plus an applicable margin:a.a base rate, which is the greatest of (i) the Canadian Reference Bank prime rate, (ii) the annual rate of interest equal to the sum of the CDOR Rate plus 1.000% and (iii) the LIBOR Market Index Rate plus 1.000%; or b.LIBOR. Borrowings by the Dutch Borrower bear interest at LIBOR plus an applicable margin.The interest rate margins on the borrowings and letters of credit are based on our leverage ratio and will range from 1.225% to 1.900% on LIBOR and swingline loans, and from 0.225% to 0.900% on Base Rate and Canadian Base Rate loans.   Borrowings under the swingline loans are based on the LIBOR Market Index Rate plus any applicable margin.
Annual facility fee, minimum (in hundredths)	0.15%
Annual facility fee, maximum (in hundredths)	0.35%
Interest rate swap [Abstract]
Incremental interest expense arising from difference between interest paid and interest received related to swap agreements	1,292,000	3,575,000
Financial and other covenants [Abstract]
Financial covenants, dividend limitation as percent of preceding year's net income, maximum (in hundredths)	50.00%
Deferred financing costs [Abstract]
Balance at beginning of year	3,451,000	2,172,000
Financing costs deferred	0	1,674,000	145,000
Write-off fully amortized deferred financing costs	(396,000)	(395,000)
Balance at end of year	3,055,000	3,451,000	2,172,000
Accumulated amortization of deferred financing costs [Abstract]
Balance at beginning of year	(1,581,000)	(1,652,000)
Amortization of deferred financing costs	(388,000)	(324,000)
Write Off Of Accumulated Amortized Deferred Financing Costs	396,000	395,000
Balance at end of year	(1,573,000)	(1,581,000)	(1,652,000)
Deferred financing costs, net of accumulated amortization	1,482,000	1,870,000
Interest Rate Swap 1, Credit Facility [Member]
Interest rate swap [Abstract]
Interest rate swap agreement, fixed interest rate (in hundredths)	1.185%
Interest rate swap agreement, notional amount	25,000,000
Interest rate swap agreement, effective date	Nov. 21, 2011
Interest rate swap agreement, termination date	Oct. 19, 2016
Interest Rate Swap 2, Credit Facility [Member]
Interest rate swap [Abstract]
Interest rate swap agreement, fixed interest rate (in hundredths)	1.185%
Interest rate swap agreement, notional amount	25,000,000
Interest rate swap agreement, effective date	Nov. 21, 2011
Interest rate swap agreement, termination date	Oct. 19, 2016
Interest Rate Swap 3, Credit Facility [Member]
Interest rate swap [Abstract]
Interest rate swap agreement, fixed interest rate (in hundredths)	1.10%
Interest rate swap agreement, notional amount	50,000,000
Interest rate swap agreement, effective date	Dec. 21, 2011
Interest rate swap agreement, termination date	Oct. 19, 2016
Interest Rate Swap 4, Credit Facility [Member]
Interest rate swap [Abstract]
Interest rate swap agreement, fixed interest rate (in hundredths)	1.05%
Interest rate swap agreement, notional amount	25,000,000
Interest rate swap agreement, effective date	Jan. 17, 2012
Interest rate swap agreement, termination date	Oct. 19, 2016
Interest Rate Swap, Credit Facility [Member]
Interest rate swap [Abstract]
Interest rate swap agreement, fixed interest rate (in hundredths)	0.99%
Interest rate swap agreement, notional amount	25,000,000
Interest rate swap agreement, effective date	Jan. 19, 2012
Interest rate swap agreement, termination date	Oct. 19, 2016
Unsecured Syndicated Senior Credit Facility [Member]
Long-term portion [Abstract]
Long-term Line of Credit, Noncurrent	230,882,000	147,300,000
Floating Rate Senior Notes [Member]
Long-term portion [Abstract]
Floating Rate Senior Notes (described below)	$ 0	$ 100,000,000
Unsecured syndicated senior credit facility [Abstract]
Debt Instrument, Maturity Date	Oct. 19, 2016