Hedging - Deferred Net Gains (Losses) in AOCI Expected to Be Reclassed into Income (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred, Year One	$ 4,736
Cash flow hedging instruments | Currency Hedge Portfolio
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred, Year One	7,535
Cash flow hedging instruments | Bunker Fuel Forward Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred, Year One	$ (2,799)