Earnings per share - Computation of Basic and Diluted Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income	$ 4,199	$ 7,918	$ 10,209	$ 8,120	$ 12,117	$ 18,329
(Income) loss attributable to noncontrolling interests	251		(317)		172	(427)
Preferred stock dividends	(7,045)		(7,045)		(14,090)	(14,090)
Earnings attributable to participating securities, basic	(2,678)		(5,552)		(7,964)	(7,148)
Earnings attributable to participating securities, diluted	(2,678)		(5,552)		(7,964)	(7,148)
Net loss available to common stockholders after allocation to participating securities, basic	(5,273)		(2,705)		(9,765)	(3,336)
Net loss available to common stockholders after allocation to participating securities, diluted	$ (5,273)		$ (2,705)		$ (9,765)	$ (3,336)
Weighted average shares outstanding - basic (in shares)	69,338		59,773		66,903	58,900
Effect of Class O Units, TSR Units, FFO Units and options to purchase Class A common stock on an "as if" converted basis (in shares)	0		0		0	0
Weighted average shares outstanding - diluted (in shares)	69,338		59,773		66,903	58,900
Basic net loss per share (in dollars per share)	$ (0.08)		$ (0.05)		$ (0.15)	$ (0.06)
Diluted net loss per share (in dollars per share)	$ (0.08)		$ (0.05)		$ (0.15)	$ (0.06)