Property and equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Property and equipment
Total property and equipment	$ 215,584	$ 198,764
Less: accumulated depreciation and amortization	(94,824)	(86,992)
Total property and equipment, net	120,760	111,772
Leasehold improvements
Property and equipment
Total property and equipment	157,656	152,627
Construction in progress
Property and equipment
Total property and equipment	28,939	20,104
Computer equipment
Property and equipment
Total property and equipment	10,170	7,909
Software
Property and equipment
Total property and equipment	18,492	17,827
Office equipment
Property and equipment
Total property and equipment	$ 327	$ 297