Investment In Unconsolidated Ventures - Summarized Income Statement Information (Detail) - USD ($) $ in Thousands	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Equity Method Investments and Joint Ventures [Abstract]
Revenues								$ 22,215		$ 65,748	$ 46,130
Earnings (Loss)								15,112		39,150	9,119
Our share of earnings (loss)	$ 2,266	$ 1,000	$ 1,529	$ 7,026	$ 1,764	$ 2,747	$ 6,362	$ 4,795	$ 10,873	$ 17,899	$ 6,123