Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total revenue	$ 50,011		$ 47,877		$ 44,959		$ 41,871		$ 36,574		$ 30,154		$ 28,666		$ 28,183		$ 184,716	$ 123,576	$ 81,334
Operating (loss) income	8,728		8,629		7,636		4,701		(40,579)		126		(2,202)		(1,992)		29,688	(44,647)	(1,032)
Net income (loss)	(5,772)		(3,111)		(3,432)		(2,955)		(47,141)		(4,581)		34,768		9,928		(15,275)	(7,028)	(2,974)
Net (loss) income available to common stockholders	$ (12,785)		$ (10,334)		$ (10,212)		$ (9,425)		$ (46,241)		$ (12,017)		$ 17,569		$ (4,990)		$ (42,759)	$ (45,679)	$ (18,985)
(Loss) earnings per common share, basic	$ (0.55)	[1]	$ (0.44)	[1]	$ (0.44)	[1]	$ (0.40)	[1]	$ (1.87)	[2]	$ (0.45)	[2]	$ 0.67	[2]	$ (0.20)	[2]	$ (1.82)	$ (1.79)	$ (0.91)
(Loss) earnings per common share, diluted	$ (0.55)	[1]	$ (0.44)	[1]	$ (0.44)	[1]	$ (0.40)	[1]	$ (1.87)	[2]	$ (0.45)	[2]	$ 0.67	[2]	$ (0.20)	[2]	$ (1.82)	$ (1.79)	$ (0.91)

[1] EPS amounts are based on weighted average common shares outstanding during the quarter and, therefore, may not agree with the EPS calculated for the year ended December 31, 2018.

[2] EPS amounts are based on weighted average common shares outstanding during the quarter and, therefore, may not agree with the EPS calculated for the year ended December 31, 2017.