Quarterly Financial Data (unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 16,301	$ 23,676	$ 50,434	$ 19,865	$ 21,891	$ 33,988	$ 30,647	$ 13,512	$ 110,276	$ 100,038	$ 96,862
Operating (loss) income	(1,343)	1,944	29,344	(541)	(1,555)	4,787	4,927	(4,582)	29,404	3,577	3,078
Net (loss) income attributable to the Company	$ (66)	$ 5,483	$ 26,195	$ 757	$ 38,503	$ 5,943	$ 10,764	$ 4,368	$ 32,369	$ 59,578	$ 15,895
Basic and diluted income per share attributable to the Company (in dollars per share)		$ 0.09	$ 0.41	$ 0.01	$ 0.58	$ 0.08	$ 0.15	$ 0.06	$ 0.52	$ 0.84	$ 0.21