Financial Assets and Liabilities - Not measured at fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial Instruments Not Measured at Fair Value
Cash and cash equivalents	$ 532,887	$ 181,415	$ 163,235	$ 75,864
Securities borrowed	1,471,172	220,005
Receivables from broker dealers and clearing organizations:
Due from prime brokers	219,573	91,476
Unsettled trades with clearing organization	112,847	21,995
Securities failed to deliver	248,088	77,915
Commissions and fees	14,021
Total receivables from broker-dealers and clearing organizations	972,018	448,728
Total Assets	7,320,006	3,692,390
Short term borrowings	27,883	25,000
Long-term borrowings	1,388,548	564,957
Securities loaned	754,687	222,203
Securities sold under agreements to repurchase	390,642
Payables to broker dealer and clearing organizations
Due to prime brokers	197,439	227,335
Securities failed to receive	51,143	5
Commissions and fees	3,068
Total payables to broker-dealers and clearing organizations	716,205	695,978
Total Liabilities	6,168,428	3,157,978
Carrying Value
Financial Instruments Not Measured at Fair Value
Cash and cash equivalents	532,887	181,415
Securities borrowed	1,471,172	220,005
Receivables from broker dealers and clearing organizations:
Total receivables from broker-dealers and clearing organizations	972,018	448,728
Total Assets	2,976,077	850,148
Short term borrowings	27,883	25,000
Long-term borrowings	1,388,548	564,957
Securities loaned	754,687	222,203
Securities sold under agreements to repurchase	390,642
Payables to broker dealer and clearing organizations
Total payables to broker-dealers and clearing organizations	716,205	695,978
Total Liabilities	3,277,965	1,508,138
Fair value measurements not measured on a recurring basis | Fair Value
Financial Instruments Not Measured at Fair Value
Cash and cash equivalents	532,887	181,415
Securities borrowed	1,471,172	220,005
Receivables from broker dealers and clearing organizations:
Total receivables from broker-dealers and clearing organizations	972,018	972,018
Total Assets	2,976,077	1,373,438
Short term borrowings	27,883	25,000
Long-term borrowings	1,465,489	564,957
Securities loaned	754,687	222,203
Securities sold under agreements to repurchase	390,642
Payables to broker dealer and clearing organizations
Total payables to broker-dealers and clearing organizations	716,205	695,978
Total Liabilities	3,354,906	1,508,138
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements not measured on a recurring basis | Fair Value
Financial Instruments Not Measured at Fair Value
Cash and cash equivalents	532,887	181,415
Receivables from broker dealers and clearing organizations:
Total receivables from broker-dealers and clearing organizations	36,513
Total Assets	569,400	181,415
Payables to broker dealer and clearing organizations
Total payables to broker-dealers and clearing organizations	2,925
Total Liabilities	2,925
Significant Other Observable Inputs (Level 2) | Fair value measurements not measured on a recurring basis | Fair Value
Financial Instruments Not Measured at Fair Value
Securities borrowed	1,471,172	220,005
Receivables from broker dealers and clearing organizations:
Total receivables from broker-dealers and clearing organizations	935,505	972,018
Total Assets	2,406,677	1,192,023
Short term borrowings	27,883	25,000
Long-term borrowings	1,465,489	564,957
Securities loaned	754,687	222,203
Securities sold under agreements to repurchase	390,642
Payables to broker dealer and clearing organizations
Total payables to broker-dealers and clearing organizations	713,280	695,978
Total Liabilities	$ 3,351,981	1,508,138
Significant Unobservable Inputs (Level 3) | Fair value measurements not measured on a recurring basis | Fair Value
Receivables from broker dealers and clearing organizations:
Total Assets		$ 0