Financial Assets and Liabilities - Measured on a recurring basis (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Fair value measurements measured on a recurring basis
Transfers of financial assets between levels	$ 0		$ 0
Assets
Financial instruments owned, at fair value	2,297,070,000	$ 2,117,579,000
Financial instruments owned and pledged	610,536,000	595,043,000
Other assets	100,811,000	98,364,000
Liabilities
Financial instruments sold, not yet purchased	2,846,453,000	2,384,598,000
Payment receivable	50,000,000
Remaining additional potential payments	56,600,000
Fair value of remaining additional potential payments	56,200,000
Equities
Assets
Financial instruments owned and pledged	571,997,000	586,251,000
U.S. and Non-U.S. government obligations
Assets
Financial instruments owned and pledged	200,000	99,000
Exchange traded notes
Assets
Financial instruments owned and pledged	38,339,000	8,693,000
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	1,031,258,000	785,185,000
Financial instruments owned and pledged	331,931,000	410,851,000
Other assets	2,154,000	1,952,000
Liabilities
Financial instruments sold, not yet purchased	1,645,535,000	874,109,000
Fair value measurements measured on a recurring basis \| Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	1,966,467,000	3,360,091,000
Financial instruments owned, at fair value, counterparty and cash collateral netting	(700,655,000)	(2,027,697,000)
Financial instruments owned and pledged	278,605,000	184,192,000
Other assets	56,179,000	55,824,000
Liabilities
Financial instruments sold, not yet purchased	1,921,382,000	3,535,480,000
Financial instruments sold, not yet purchased, at fair value, counterparty and cash collateral netting	(720,464,000)	(2,024,991,000)
Fair value measurements measured on a recurring basis \| Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned and pledged	0	0
Other assets	42,478,000	40,588,000
Liabilities
Financial instruments sold, not yet purchased	0	0
Fair value measurements measured on a recurring basis \| Currency forwards \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased	0	0
Fair value measurements measured on a recurring basis \| Currency forwards \| Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	702,889,000	2,045,487,000
Financial instruments owned, at fair value, counterparty and cash collateral netting	(700,655,000)	(2,027,697,000)
Liabilities
Financial instruments sold, not yet purchased	725,422,000	2,032,017,000
Financial instruments sold, not yet purchased, at fair value, counterparty and cash collateral netting	(720,464,000)	(2,024,991,000)
Fair value measurements measured on a recurring basis \| Currency forwards \| Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased	0	0
Fair value measurements measured on a recurring basis \| Options \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	9,210,000	7,045,000
Liabilities
Financial instruments sold, not yet purchased	15,548,000	5,839,000
Fair value measurements measured on a recurring basis \| Options \| Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased	0	0
Fair value measurements measured on a recurring basis \| Options \| Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased	0	0
Fair value measurements measured on a recurring basis \| Equities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	956,518,000	758,596,000
Financial instruments owned and pledged	309,313,000	410,670,000
Liabilities
Financial instruments sold, not yet purchased	1,584,268,000	847,816,000
Fair value measurements measured on a recurring basis \| Equities \| Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	1,139,469,000	1,167,995,000
Financial instruments owned and pledged	262,684,000	175,581,000
Liabilities
Financial instruments sold, not yet purchased	1,132,360,000	1,355,616,000
Fair value measurements measured on a recurring basis \| Equities \| Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned and pledged	0	0
Liabilities
Financial instruments sold, not yet purchased	0	0
Fair value measurements measured on a recurring basis \| U.S. and Non-U.S. government obligations \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	800,000	5,968,000
Financial instruments owned and pledged	200,000	99,000
Liabilities
Financial instruments sold, not yet purchased	1,998,000
Fair value measurements measured on a recurring basis \| U.S. and Non-U.S. government obligations \| Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	18,207,000	16,815,000
Financial instruments owned and pledged	0	0
Liabilities
Financial instruments sold, not yet purchased	25,207,000
Fair value measurements measured on a recurring basis \| U.S. and Non-U.S. government obligations \| Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned and pledged	0	0
Liabilities
Financial instruments sold, not yet purchased	0
Fair value measurements measured on a recurring basis \| Corporate Bonds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased	40,007,000
Fair value measurements measured on a recurring basis \| Corporate Bonds \| Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	75,534,000	60,975,000
Liabilities
Financial instruments sold, not yet purchased	0
Fair value measurements measured on a recurring basis \| Corporate Bonds \| Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased	0
Fair value measurements measured on a recurring basis \| Exchange traded notes \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	64,730,000	13,576,000
Financial instruments owned and pledged	22,418,000	82,000
Liabilities
Financial instruments sold, not yet purchased	3,714,000	1,514,000
Fair value measurements measured on a recurring basis \| Exchange traded notes \| Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	30,368,000	68,819,000
Financial instruments owned and pledged	15,921,000	8,611,000
Liabilities
Financial instruments sold, not yet purchased	38,393,000	54,248,000
Fair value measurements measured on a recurring basis \| Exchange traded notes \| Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned and pledged	0	0
Liabilities
Financial instruments sold, not yet purchased	0	0
Fair value measurements measured on a recurring basis \| Equity investment \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Other assets	0	0
Fair value measurements measured on a recurring basis \| Equity investment \| Significant Other Observable Inputs (Level 2)
Assets
Other assets	0	0
Fair value measurements measured on a recurring basis \| Equity investment \| Significant Unobservable Inputs (Level 3)
Assets
Other assets	42,478,000	40,588,000
Fair value measurements measured on a recurring basis \| Exchange stock \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Other assets	2,154,000	1,952,000
Fair value measurements measured on a recurring basis \| Exchange stock \| Significant Other Observable Inputs (Level 2)
Assets
Other assets	0	0
Fair value measurements measured on a recurring basis \| Exchange stock \| Significant Unobservable Inputs (Level 3)
Assets
Other assets	0	0
Fair value measurements measured on a recurring basis \| Other \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Other assets	0	0
Fair value measurements measured on a recurring basis \| Other \| Significant Other Observable Inputs (Level 2)
Assets
Other assets	56,179,000	55,824,000
Fair value measurements measured on a recurring basis \| Other \| Significant Unobservable Inputs (Level 3)
Assets
Other assets	0	0
Fair Value \| Fair value measurements measured on a recurring basis
Assets
Financial instruments owned, at fair value	2,297,070,000	2,117,579,000
Financial instruments owned and pledged	610,536,000	595,043,000
Other assets	100,811,000	98,364,000
Liabilities
Financial instruments sold, not yet purchased	2,846,453,000	2,384,598,000
Fair Value \| Fair value measurements measured on a recurring basis \| Currency forwards
Assets
Financial instruments owned, at fair value	2,234,000	17,790,000
Liabilities
Financial instruments sold, not yet purchased	4,958,000	7,026,000
Fair Value \| Fair value measurements measured on a recurring basis \| Options
Assets
Financial instruments owned, at fair value	9,210,000	7,045,000
Liabilities
Financial instruments sold, not yet purchased	15,548,000	5,839,000
Fair Value \| Fair value measurements measured on a recurring basis \| Equities
Assets
Financial instruments owned, at fair value	2,095,987,000	1,926,591,000
Financial instruments owned and pledged	571,997,000	586,251,000
Liabilities
Financial instruments sold, not yet purchased	2,716,628,000	2,203,432,000
Fair Value \| Fair value measurements measured on a recurring basis \| U.S. and Non-U.S. government obligations
Assets
Financial instruments owned, at fair value	19,007,000	22,783,000
Financial instruments owned and pledged	200,000	99,000
Liabilities
Financial instruments sold, not yet purchased	27,205,000
Fair Value \| Fair value measurements measured on a recurring basis \| Corporate Bonds
Assets
Financial instruments owned, at fair value	75,534,000	60,975,000
Liabilities
Financial instruments sold, not yet purchased	40,007,000
Fair Value \| Fair value measurements measured on a recurring basis \| Exchange traded notes
Assets
Financial instruments owned, at fair value	95,098,000	82,395,000
Financial instruments owned and pledged	38,339,000	8,693,000
Liabilities
Financial instruments sold, not yet purchased	42,107,000	55,762,000
Fair Value \| Fair value measurements measured on a recurring basis \| Equity investment
Assets
Other assets	42,478,000	40,588,000
Fair Value \| Fair value measurements measured on a recurring basis \| Exchange stock
Assets
Other assets	2,154,000	1,952,000
Fair Value \| Fair value measurements measured on a recurring basis \| Other
Assets
Other assets	$ 56,179,000	$ 55,824,000
SBI
Liabilities
Discount rate applied to cash flow forecasts to determine initial fair value of equity method investment.	15.00%
Market approach average price/earnings multiples of comparable companies to corroborate the income approach	14
Time period of projected income to determine fair value of equity method investment	5 years
Maximum
Liabilities
Annual payment receivable	$ 6,600,000