Variable interest entities - Carrying amounts and maximum exposure to loss for E&O joint venture (Details) - Variable Interest Entity, Not Primary Beneficiary - Eastern and Oriental Express Ltd. - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Carrying amounts, Total	$ 8,310	$ 7,589
Maximum exposure	8,310	7,589
Investment
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Investment/Due from unconsolidated company, Carrying amounts	2,740	2,818
Maximum exposure	2,740	2,818
Due from unconsolidated company
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Investment/Due from unconsolidated company, Carrying amounts	5,570	4,771
Maximum exposure	5,570	4,771
Guarantees
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Guarantees/Contingent guarantees, Carrying amounts	0	0
Maximum exposure	0	0
Contingent guarantees
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Guarantees/Contingent guarantees, Carrying amounts	0	0
Maximum exposure	$ 0	$ 0