Acquisition of KCG Holdings, Inc. - Intangible assets (Details) - USD ($) $ in Thousands	Jul. 20, 2017	Dec. 31, 2017	Dec. 31, 2016
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		$ 111,224	$ 992
Goodwill		844,883	715,379
Deferred tax asset		125,760	193,859
Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		47,178
Favorable leases
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		5,487
VFH
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Deferred tax asset		124,631	192,961
KCG
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 174,995	50,800
Goodwill	128,286	128,286
Total	303,281
Goodwill deductible for tax purposes	0
Deferred tax asset	23,900	43,500
KCG | Technology
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	67,700
KCG | Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	94,000
KCG | Trade names
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 1,000
Amortization Years	10 years
KCG | Favorable leases
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 5,895
KCG | Exchange memberships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 6,400
Minimum | KCG | Technology
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	1 year
Minimum | KCG | Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	13 years
Minimum | KCG | Favorable leases
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	2 years
Maximum | KCG | Technology
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	6 years
Maximum | KCG | Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	17 years
Maximum | KCG | Favorable leases
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	15 years
Market Making
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Goodwill	$ 96,200	755,292	657,985
Execution Services
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Goodwill		$ 89,591	$ 57,394
Technology and Execution Services
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Goodwill	$ 32,100