Cash, cash equivalents, and restricted cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash, cash equivalents, and restricted cash [Line Items]
Cash and cash equivalents	$ 221,645	$ 254,381
Restricted cash	37,337	22,805
Cash, cash equivalents, and restricted cash	258,982	277,186	$ 169,529	$ 141,366
Funds held in trust under the terms of certain secured notes payable
Cash, cash equivalents, and restricted cash [Line Items]
Restricted cash	15,558	12,301
Funds held in escrow related to construction projects and investing activities
Cash, cash equivalents, and restricted cash [Line Items]
Restricted cash	17,048	4,546
Other restricted cash
Cash, cash equivalents, and restricted cash [Line Items]
Restricted cash	$ 4,731	$ 5,958