Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 1,100		$ 4,800
Cash collateral received	3,700		4,500
Loss on equity options activity	7,600	$ 4,200
Exchange-traded options \| Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0
Long exchange-traded equity options	3,000		2,600
Exchange-traded options \| Short
Derivative [Line Items]
Cash collateral received	0		0
Short exchange-traded equity options, included in securities sold not yet purchased	$ 2,900		$ 3,800

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 55
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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

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Aggregate net gain and losses (whether realized or unrealized) on all derivative instruments held for trading purposes whether or not settled physically.

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