Debt and leases (credit facility) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Debt and leases [Abstract]
Amounts drawn on credit facility	$ 0	$ 7,000
Credit facility, expiration date	March 2013
Commitment fees, minimum (in hundredths)	0.40%
Commitment fees, maximum (in hundredths)	0.50%
Weighted-average interest rate on amount outstanding (in hundredths)		5.25%
Ratio of EBITDA to interest expense	2 to 1
Credit facility availability [Abstract]
Credit facility commitment	200,000
Credit facility outstanding [Abstract]
Daily average amount outstanding	21,655	49,957	69,267
Weighted-average interest rate (in hundredths)	3.03%	3.20%	0.76%
Credit Facility [Member]
Credit facility availability [Abstract]
Credit facility commitment	200,000
Outstanding letters of credit	(8,551)
Net available for borrowing	$ 191,449