Financial Assets and Liabilities - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair value measurements measured on a recurring basis
Transfers of financial assets between levels	$ 0	$ 0
Assets
Financial instruments owned, at fair value	1,038,039	1,307,933
Financial instruments owned, pledged as collateral	259,175	236,375
Other assets: exchange stock	5,984	8,205
Liabilities
Financial instruments sold, not yet purchased, at fair value	979,090	1,037,634
Equity securities
Assets
Financial instruments owned, pledged as collateral	232,731	219,159
Exchange traded notes
Assets
Financial instruments owned, pledged as collateral	26,444	17,216
Fair value measurements measured on a recurring basis | Total Fair Value
Assets
Financial instruments owned, at fair value	1,038,039	1,307,933
Financial instruments owned, pledged as collateral	259,175	236,375
Other assets: exchange stock	5,984	8,205
Liabilities
Financial instruments sold, not yet purchased, at fair value	979,090	1,037,634
Fair value measurements measured on a recurring basis | Total Fair Value | Currency forwards
Assets
Financial instruments owned, at fair value	9,580	8
Liabilities
Financial instruments sold, not yet purchased, at fair value		16,191
Fair value measurements measured on a recurring basis | Total Fair Value | Options
Assets
Financial instruments owned, at fair value	168	321
Liabilities
Financial instruments sold, not yet purchased, at fair value	163	79
Fair value measurements measured on a recurring basis | Total Fair Value | Interest rate swaps
Assets
Financial instruments owned, at fair value	311
Liabilities
Financial instruments sold, not yet purchased, at fair value	319	12
Fair value measurements measured on a recurring basis | Total Fair Value | Equity securities
Assets
Financial instruments owned, at fair value	948,091	1,233,698
Financial instruments owned, pledged as collateral	232,731	219,159
Liabilities
Financial instruments sold, not yet purchased, at fair value	936,851	907,732
Fair value measurements measured on a recurring basis | Total Fair Value | US and non-US government obligations
Assets
Financial instruments owned, at fair value	10,513	8,222
Liabilities
Financial instruments sold, not yet purchased, at fair value	3,996	21,107
Fair value measurements measured on a recurring basis | Total Fair Value | Exchange traded notes
Assets
Financial instruments owned, at fair value	69,376	65,684
Financial instruments owned, pledged as collateral	26,444	17,216
Liabilities
Financial instruments sold, not yet purchased, at fair value	37,761
Fair value measurements measured on a recurring basis | Total Fair Value | Exchange stock
Assets
Other assets: exchange stock	5,984	8,205
Liabilities
Financial instruments sold, not yet purchased, at fair value		92,513
Fair value measurements measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	985,224	1,282,216
Financial instruments owned, pledged as collateral	259,175	236,375
Other assets: exchange stock	5,984	8,205
Liabilities
Financial instruments sold, not yet purchased, at fair value	976,828	973,456
Fair value measurements measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Assets
Financial instruments owned, at fair value	915,848	1,216,532
Financial instruments owned, pledged as collateral	232,731	219,159
Liabilities
Financial instruments sold, not yet purchased, at fair value	935,071	859,836
Fair value measurements measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US and non-US government obligations
Liabilities
Financial instruments sold, not yet purchased, at fair value	3,996	21,107
Fair value measurements measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange traded notes
Assets
Financial instruments owned, at fair value	69,376	65,684
Financial instruments owned, pledged as collateral	26,444	17,216
Liabilities
Financial instruments sold, not yet purchased, at fair value	37,761
Fair value measurements measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange stock
Assets
Other assets: exchange stock	5,984	8,205
Liabilities
Financial instruments sold, not yet purchased, at fair value		92,513
Fair value measurements measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	838,670	1,655,346
Liabilities
Financial instruments sold, not yet purchased, at fair value	748,276	1,693,807
Fair value measurements measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Currency forwards
Assets
Financial instruments owned, at fair value	795,435	1,629,637
Liabilities
Financial instruments sold, not yet purchased, at fair value	746,014	1,645,820
Fair value measurements measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Options
Assets
Financial instruments owned, at fair value	168	321
Liabilities
Financial instruments sold, not yet purchased, at fair value	163	79
Fair value measurements measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Interest rate swaps
Assets
Financial instruments owned, at fair value	311
Liabilities
Financial instruments sold, not yet purchased, at fair value	319	12
Fair value measurements measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Financial instruments owned, at fair value	32,243	17,166
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,780	47,896
Fair value measurements measured on a recurring basis | Significant Other Observable Inputs (Level 2) | US and non-US government obligations
Assets
Financial instruments owned, at fair value	10,513	8,222
Fair value measurements measured on a recurring basis | Counterparty and Cash Collateral Netting
Assets
Financial instruments owned, at fair value	(785,855)	(1,629,629)
Liabilities
Financial instruments sold, not yet purchased, at fair value	(746,014)	(1,629,629)
Fair value measurements measured on a recurring basis | Counterparty and Cash Collateral Netting | Currency forwards
Assets
Financial instruments owned, at fair value	(785,855)	(1,629,629)
Liabilities
Financial instruments sold, not yet purchased, at fair value	$ (746,014)	$ (1,629,629)