Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income (loss)	$ 9,338	$ (167)	$ (3,753)	$ 5,028	$ (11,347)	$ (19,938)	$ (8,398)	$ (8,265)	$ 10,446	$ (47,948)	$ (33,092)
Noncontrolling interest									3,751	14,353	14,965
Dividends on preferred shares									40,237	22,276	18,928
Net loss attributable to common shareholders	$ 2,391	$ (21,152)	$ (10,404)	$ (4,377)	$ (20,474)	$ (28,616)	$ (17,697)	$ (17,790)	$ (33,542)	$ (84,577)	$ (66,985)
Denominator:
Weighted-average shares outstanding–basic and diluted (in shares)									234,010,168	210,600,111	196,348,757
Net loss per share—basic and diluted:
Net loss attributable to common shareholders per share - basic and diluted (in dollars per share)					$ (0.10)	$ (0.14)	$ (0.08)	$ (0.08)	$ (0.14)	$ (0.40)	$ (0.34)