Hedging - Deferred Net Gains (Losses) in AOCI Expected to Be Reclassed into Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
2014	$ (5,384)
2015	(464)
Deferred net gains (losses) in Accumulated other comprehensive income (loss) expected to be reclassified into income	(5,848)	(4,517)
Cash flow hedging instruments | Currency Hedge Portfolio
Derivative Instruments, Gain (Loss) [Line Items]
2014	(4,413)
2015	(661)
Deferred net gains (losses) in Accumulated other comprehensive income (loss) expected to be reclassified into income	(5,074)
Cash flow hedging instruments | Bunker Fuel Forward Contracts
Derivative Instruments, Gain (Loss) [Line Items]
2014	(971)
2015	197
Deferred net gains (losses) in Accumulated other comprehensive income (loss) expected to be reclassified into income	$ (774)