Quarterly Financial Information - Summary of Unaudited Interim Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 1,961	$ 2,287	$ 2,032	$ 1,610	$ 1,223	$ 1,284	$ 1,389	$ 1,500	$ 1,292	$ 1,488	$ 1,470	$ 1,750	$ 7,890	$ 5,396	$ 6,000
Net income (loss) attributable to common and participating stockholders	$ (13,544)	$ 33,911	$ (25,824)	$ 13,289	$ 135,886	$ (21,145)	$ 17,909	$ 51,050	$ 5,568	$ (8,419)	$ (77,983)	$ 1,253	$ 7,832	$ 183,700	$ (79,581)
Basic net income (loss) per share attributable to common stockholders	$ (0.07)	$ 0.06	$ (0.13)	$ 0.02	$ 0.24	$ (0.10)	$ 0.04	$ 0.11	$ 0.01	$ (0.04)	$ (0.39)	$ 0.00	$ 0.01	$ 0.33	$ (0.39)
Diluted net income (loss) per share attributable to common stockholders	$ (0.07)	$ 0.06	$ (0.13)	$ 0.02	$ 0.23	$ (0.10)	$ 0.03	$ 0.10	$ 0.01	$ (0.04)	$ (0.39)	$ 0.00	$ 0.01	$ 0.30	$ (0.39)