Commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Commitments and contingencies
Rent expense for leases from governmental authorities	$ 16,760	$ 15,511	$ 12,499
Interest expense associated with the capital leases	11	8	30
Rent expense for leases of office facilities	1,209	1,323	1,325
Sublease income included in rent expenses	52	32
Capital Leases
2013	42
2014	42
2015	42
2016	52
Capital leases minimum lease payments	178
Current portion	42	205
Non-current portion	136	197
Operating Leases and Airport Guarantees
2013	7,409
2014	6,068
2015	4,985
2016	4,860
2017	4,727
Thereafter	21,602
Minimum lease payments	$ 49,651
Employment contracts
Number of officers with whom the entity entered into employment contracts	2