Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Trading income, net	$ 766,027	$ 665,465	$ 757,455
Interest and dividends income	50,407	26,419	28,136
Commissions, net and technology services	116,503	10,352	10,622
Other, net	95,045	36
Total revenue	1,027,982	702,272	796,213
Operating Expenses:
Brokerage, exchange and clearance fees, net	256,926	221,214	232,469
Communication and data processing	131,506	71,001	68,647
Employee compensation and payroll taxes	177,489	85,295	88,026
Payments for order flow	27,727
Interest and dividends expense	91,993	56,557	52,423
Operations and administrative	65,137	23,039	25,991
Depreciation and amortization	47,327	29,703	33,629
Amortization of purchased intangibles and acquired capitalized software	15,447	211	211
Debt issue costs related to debt refinancing	10,460	5,579
Transaction advisory fees and expenses	25,270
Reserve for legal matter	657		5,440
Charges related to share-based compensation at IPO	772	1,755	44,194
Financing interest expense on long-term borrowings	64,107	28,327	29,254
Total operating expenses	914,818	522,681	580,284
Income (loss) before income taxes and noncontrolling interest	113,164	179,591	215,929
Provision for (benefit from) income taxes	94,266	21,251	18,439
Net income (loss)	18,898	158,340	197,490
Noncontrolling interest	(15,959)	(125,360)	(176,603)
Net income (loss) available for common stockholders	$ 2,939	$ 32,980	$ 20,887
Earnings (loss) per share
Basic (in dollars per share)	$ 0.03	$ 0.83	$ 0.60
Diluted (in dollars per share)	$ 0.03	$ 0.83	$ 0.59
Weighted average common shares outstanding
Basic (in shares)	62,579,147	38,539,091	34,964,312
Diluted (in shares)	62,579,147	38,539,091	35,339,585
Comprehensive income
Net income (loss)	$ 18,898	$ 158,340	$ 197,490
Other comprehensive income (loss)
Foreign currency translation adjustment, net of taxes	9,117	(1,165)	(4,255)
Comprehensive income (loss)	28,015	157,175	193,235
Less: Comprehensive income (loss) attributable to non-controlling interests	(21,833)	(124,546)	(172,249)
Comprehensive income (loss) attributable to common stockholders	$ 6,182	$ 32,629	$ 20,986