Commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	3 Months Ended			1 Months Ended	3 Months Ended
	Aug. 09, 2012 court_case	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Jun. 07, 2011 Pending Litigation Minimum	Mar. 31, 2014 Letter of Credit	Mar. 31, 2013 Letter of Credit
Loss Contingencies [Line Items]
Letters of credit outstanding		$ 920		$ 920
Interest paid		18,794	16,787			0	0
Damages sought					$ 5,000
Number of court cases	2