Investment in Securities, Fair Values and Gains and Losses of All Derivatives Held (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Asset derivatives fair values	$ 143	$ 800	$ 143	$ 800	$ 896
Liability derivatives fair value	41,621	7,009	41,621	7,009	6,105
Derivative Instruments, Gain (Loss) [Line Items]
Net gain/(loss) on derivative	2,012	1,240	2,084	1,132
Net Gain/(Loss) From Investments [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain/(loss) on derivative	1,985	482	1,885	541
Net Gain/(Loss) From Investments [Member] | Equity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain/(loss) on derivative	27	758	199	591
Derivatives Designated As Hedging Instruments Under FASB ASC 815-20 [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives fair values	0	0	0	0	0
Liability derivatives fair value	36,354	0	36,354	0	0
Derivatives Designated As Hedging Instruments Under FASB ASC 815-20 [Member] | Receivable From Brokers [Member] | Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives fair values	0	0	0	0	0
Derivatives Designated As Hedging Instruments Under FASB ASC 815-20 [Member] | Payable To Brokers [Member] | Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives fair value	36,354	0	36,354	0	0
Derivatives Not Designated As Hedging Instruments Under Fasb Asc 815-20 [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives fair values	143	800	143	800	896
Liability derivatives fair value	5,267	7,009	5,267	7,009	6,105
Derivatives Not Designated As Hedging Instruments Under Fasb Asc 815-20 [Member] | Receivable From Brokers [Member] | Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives fair values	0	0	0	0	0
Derivatives Not Designated As Hedging Instruments Under Fasb Asc 815-20 [Member] | Payable To Brokers [Member] | Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives fair value	5,172	6,343	5,172	6,343	5,470
Derivatives Not Designated As Hedging Instruments Under Fasb Asc 815-20 [Member] | Investments In Securities [Member] | Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives fair values	143	800	143	800	896
Derivatives Not Designated As Hedging Instruments Under Fasb Asc 815-20 [Member] | Securities Sold, Not yet Purchased [Member] | Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives fair value	$ 95	$ 666	$ 95	$ 666	$ 635