Subsequent event (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended	1 Months Ended
	Dec. 31, 2011	Feb. 29, 2012 Modification of Credit Facility [Member]
Subsequent Event [Line Items]
Credit facility commitment	$ 200,000	$ 200,000
Credit facility, date of expiration	March 2013	February 2017
Commitment fees, minimum (in hundredths)	0.40%	0.20%
Commitment fees, maximum (in hundredths)	0.50%	0.45%