Hedging - Deferred Net Gains (Losses) in AOCI Expected to Be Reclassed into Income (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
2013	$ (7,678)
2014	58
2015	(838)
Deferred net gains (losses) in Accumulated other comprehensive income (loss) expected to be reclassified into income	(8,458)
Cash flow hedging instruments | Currency Hedge Portfolio
Derivative Instruments, Gain (Loss) [Line Items]
2013	(9,088)
2014	2,510
2015	0
Deferred net gains (losses) in Accumulated other comprehensive income (loss) expected to be reclassified into income	(6,578)
Cash flow hedging instruments | Bunker Fuel Forward Contracts
Derivative Instruments, Gain (Loss) [Line Items]
2013	1,410
2014	(2,452)
2015	(838)
Deferred net gains (losses) in Accumulated other comprehensive income (loss) expected to be reclassified into income	$ (1,880)