Investments in and advances to unconsolidated joint ventures - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Equity Method Investments [Line Items]
Revenues	$ 35,171	$ 70,104	$ 61,791	$ 112,061
Cost of Sales and Expenses	35,825	59,909	63,309	99,725
Net (loss) income of unconsolidated joint ventures	(654)	10,195	(1,518)	12,336
Equity in net income of unconsolidated joint ventures reflected in the accompanying consolidated statements of operations	$ 201	$ 3,947	$ 507	$ 3,940