Hedging - Derivative Assets and Liabilities Table (Details) - Designated as Hedging Instrument [Member] - USD ($) $ in Thousands		3 Months Ended
		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Currency Hedge Portfolio
Change in Fair Value of Derivative Assets and Liabilities [Roll Forward]
Balance at beginning of year		$ 7,684	$ 24,979	$ 23,735	$ (511)	$ (3,831)	$ (5,014)
Realized (gains) losses included in net income		7,727	(13,659)	(16,773)	1,308	1,414	1,229
Purchases, net	[1]	0	0	0	149	0	277
Changes in fair value		43	(3,636)	18,017	23,513	1,906	(323)
Balance at end of year		0	7,684	24,979	21,843	(511)	(3,831)
Bunker Fuel Forward Contracts
Change in Fair Value of Derivative Assets and Liabilities [Roll Forward]
Balance at beginning of year		(2,810)	(7,319)	(15,990)	946	(634)	988
Realized (gains) losses included in net income		(3,760)	3,465	8,775	(822)	(61)	(175)
Purchases, net	[1]	0	0	0	0	0	0
Changes in fair value		(950)	1,044	(104)	(5,141)	1,641	(1,447)
Balance at end of year		$ 0	$ (2,810)	$ (7,319)	$ (3,373)	$ 946	$ (634)

[1] Purchases (sales) represent the cash premiums paid upon the purchase of euro put options or received upon the sale of euro call options. Bunker fuel and currency forward contracts require no up-front cash payment and have an initial fair value of zero; settlements on the forward contracts (swaps) occur upon their maturity