Derivative Contracts - Outstanding Forward Contracts (Details) - Not Designated as Hedging Instrument - Forward contracts - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Contract value	$ 4,951	$ 908
Fair value	5,016	944
Effect on income (loss)	65	36
0 to 30
Derivative [Line Items]
Contract value	2,790	750
Fair value	2,840	780
Effect on income (loss)	$ 50	$ 30
Contract days until maturity, lower range	0 days	0 days
Contract days until maturity, higher range	30 days	30 days
31 to 60
Derivative [Line Items]
Contract value	$ 1,966	$ 90
Fair value	1,979	93
Effect on income (loss)	$ 13	$ 3
Contract days until maturity, lower range	31 days	31 days
Contract days until maturity, higher range	60 days	60 days
61 to 90
Derivative [Line Items]
Contract value	$ 195	$ 0
Fair value	197	0
Effect on income (loss)	$ 2	$ 0
Contract days until maturity, lower range	61 days	61 days
Contract days until maturity, higher range	90 days	90 days
91 to 120
Derivative [Line Items]
Contract value	$ 0	$ 68
Fair value	0	71
Effect on income (loss)	$ 0	$ 3
Contract days until maturity, lower range	91 days	91 days
Contract days until maturity, higher range	120 days	120 days
121 to 180
Derivative [Line Items]
Contract value		$ 0
Fair value		0
Effect on income (loss)		$ 0
Contract days until maturity, lower range		121 days
Contract days until maturity, higher range		180 days
More than 120
Derivative [Line Items]
Contract value	$ 0
Fair value	0
Effect on income (loss)	$ 0
Contract days until maturity, higher range	120 days
More than 180
Derivative [Line Items]
Contract value		$ 0
Fair value		0
Effect on income (loss)		$ 0
Contract days until maturity, higher range		180 days