Schedule of Amounts Due to Affiliate (Detail) - USD ($) $ in Thousands		4 Months Ended	5 Months Ended	12 Months Ended
		Jul. 31, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Beginning balance				$ 34,972
Ending balance				50,186	$ 34,972
Sonatide Marine, Ltd.
Schedule of Equity Method Investments [Line Items]
Beginning balance		$ 132,857	$ 123,899	29,347	99,448
Plus commissions payable to Sonatide		3,330	3,928	4,937	5,502
Plus amounts paid by Sonatide on behalf of the company		9,458	12,044	9,654	14,778
Less commissions paid to Sonatide			(5,023)	(5,961)	(13,906)
Less amounts used to offset Due from Sonatide obligations	[1]	(21,453)	(33,607)	(10,551)	(78,993)
Other		(293)	(1,793)	4,049	2,518
Ending balance		$ 123,899	$ 99,448	$ 31,475	$ 29,347

[1]   (A) We reduced the respective due from affiliates and due to affiliates balances each period through netting transactions based on agreement with the joint venture.