Commitments and contingent obligations - Schedule of Outstanding Commitments for Installment Payments for Vessels (Detail) $ in Thousands	Dec. 31, 2015 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2016	$ 373,247
2017	293,900
Purchase obligations for additional vessels	$ 667,147

Minimum amount of purchase arrangement maturing in the next fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

No definition available.

Name:	us-gaap_PurchaseObligationDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant