Securities Owned and Securities Sold, but not yet Purchased (and Fair Value Measurement) - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($)	Mar. 31, 2018	Apr. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liquid investments, original maturities		P90D
Number of AFS securities determined to be other than temporarily impaired | Securities		0
Acquisition payable			$ 3,942,000
Acquisition payment	$ 5,993,000	$ 6,738,000	2,185,000
Acquisition payable		$ 3,537,000
Boost [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of issued and outstanding membership interest acquired					75.00%
Boost [Member] | Scenario, Forecast [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of issued and outstanding membership interest acquired				25.00%
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Acquisition payable			3,942,000
Estimated fair value of acquisition payable			$ 9,900,000
Discounted cash flows, discount rate			27.50%
Level 3 [Member] | ETP [Member] | Europe [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Projected AUM			$ 6,000,000,000
Level 3 [Member] | ETP [Member] | Europe [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Projected AUM			$ 1,000,000,000