Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 42,660	$ 32,856	$ 35,546	$ 16,023	$ 16,301	$ 23,676	$ 50,434	$ 19,865	$ 127,085	$ 110,276	$ 100,038
Operating income (loss)	15,196	7,420	10,868	(2,161)	(1,343)	1,944	29,344	(541)	31,323	29,404	3,577
Net (loss) income attributable to the Company	$ 8,688	$ 5,717	$ 10,373	$ 1,997	$ (66)	$ 5,483	$ 26,195	$ 757	$ 26,775	$ 32,369	$ 59,578
Basic and diluted income per share attributable to the Company (in dollars per share)	$ 0.15	$ 0.10	$ 0.17	$ 0.03		$ 0.09	$ 0.41	$ 0.01	$ 0.45	$ 0.52	$ 0.84