Variable interest entities - Carrying amounts and maximum exposure to loss for Peru rail joint venture (Details) (Variable Interest Entity, Not Primary Beneficiary, Peruvian rail joint venture, USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 42,593	$ 43,052
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	62,074	63,703
Investment
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	38,396	38,095
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	38,396	38,095
Due from unconsolidated company
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	4,197	4,957
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	4,197	4,957
Guarantees
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Liabilities	0	0
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	5,486	5,920
Contingent guarantees
Schedule Of Unconsolidated Variable Interest Entity Asset Carrying Value and Maximum Exposure to Loss [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Liabilities	0	0
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	$ 13,995	$ 14,731