Summary of Operations in Angola (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2020 USD ($) Vessel	Mar. 31, 2019 USD ($) Vessel
Schedule of Equity Method Investments [Line Items]
Revenues of Angolan operations (in thousands) \| $	$ 116,368	$ 122,149
ANGOLA
Schedule of Equity Method Investments [Line Items]
Revenues of Angolan operations (in thousands) \| $	$ 12,137	$ 15,000
Percent of consolidated vessel revenues	11.00%	13.00%
Number of company owned vessels in Angola \| Vessel	28	35
Number of stacked company owned vessels in Angola \| Vessel	11	14

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration