Segment Reporting - Information about Unconsolidated Affiliates (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Other Significant Reconciling Item [Line Items]
Loss from Investments in Unconsolidated Affiliates	$ (14,228)	$ (4,090)	$ (21,486)
Investments in Unconsolidated Affiliates	7,501	17,023
Operating Segments [Member] | AEA [Member]
Segment Reporting Other Significant Reconciling Item [Line Items]
Loss from Investments in Unconsolidated Affiliates	(6,407)	(5,082)	(6,255)
Investments in Unconsolidated Affiliates	748	2,449
Operating Segments [Member] | ASA [Member]
Segment Reporting Other Significant Reconciling Item [Line Items]
Loss from Investments in Unconsolidated Affiliates	(7,312)	1,167	(15,137)
Investments in Unconsolidated Affiliates	6,753	14,064
Corporate and Other [Member]
Segment Reporting Other Significant Reconciling Item [Line Items]
Loss from Investments in Unconsolidated Affiliates	$ (509)	(175)	$ (94)
Investments in Unconsolidated Affiliates		$ 510