Derivative Contracts (Details 2) (Not Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ (27,793)	$ 169	$ (38,691)	$ 1,309
Forward Contracts [Member]
Derivative [Line Items]
Contract value	3,924		3,924		1,304
Fair value	3,899		3,899		1,294
Derivative, Gain (Loss) on Derivative, Net	(158)	(32)	(25)	(59)	(10)
Forward Contracts [Member] | 0 to 30 [Member]
Derivative [Line Items]
Contract value	1,133		1,133
Fair value	1,139		1,139
Derivative, Gain (Loss) on Derivative, Net			6		0
Contract days until maturity, lower range			0 days		0 days
Contract days until maturity, higher range			30 days		30 days
Forward Contracts [Member] | 31 to 60 [Member]
Derivative [Line Items]
Contract value	1,058		1,058		581
Fair value	1,039		1,039		577
Derivative, Gain (Loss) on Derivative, Net			(19)		(4)
Contract days until maturity, lower range			31 days		31 days
Contract days until maturity, higher range			60 days		60 days
Forward Contracts [Member] | 61 to 90 [Member]
Derivative [Line Items]
Contract value	945		945		341
Fair value	939		939		339
Derivative, Gain (Loss) on Derivative, Net			(6)		(2)
Contract days until maturity, lower range			61 days		61 days
Contract days until maturity, higher range			90 days		90 days
Forward Contracts [Member] | 91 to 120 [Member]
Derivative [Line Items]
Contract value	272		272		212
Fair value	271		271		210
Derivative, Gain (Loss) on Derivative, Net			(1)		(2)
Contract days until maturity, lower range			91 days		91 days
Contract days until maturity, higher range			120 days		120 days
Forward Contracts [Member] | 121 to 180 [Member]
Derivative [Line Items]
Contract value	516		516		170
Fair value	511		511		168
Derivative, Gain (Loss) on Derivative, Net			(5)		(2)
Contract days until maturity, lower range			121 days		121 days
Contract days until maturity, higher range			180 days		180 days
Forward Contracts [Member] | More than 180 [Member]
Derivative [Line Items]
Contract value	0		0		0
Fair value	0		0
Derivative, Gain (Loss) on Derivative, Net			$ 0		$ 0
Contract days until maturity, higher range			180 days		180 days