Collateralized Transactions - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Repurchase agreements	$ 198,000
Securities loaned	66,318,000	$ 202,343,000
Minimum [Member]
Securities Financing Transaction [Line Items]
Financial covenants including maintaining minimum cash and cash equivalents	15,000,000
CF & Co [Member]
Securities Financing Transaction [Line Items]
Repurchase agreements	$ 200,000	0
Repurchase agreements maturity date	Aug. 31, 2023
Securities loaned	$ 66,300,000	202,300,000
CF & Co [Member] \| Securities Financing Transaction, Fair Value [Member]
Securities Financing Transaction [Line Items]
Securities loaned	$ 66,300,000	$ 204,100,000
CF & Co [Member] \| Minimum [Member]
Securities Financing Transaction [Line Items]
Stated interest rate	2.70%	1.90%
CF & Co [Member] \| Maximum [Member]
Securities Financing Transaction [Line Items]
Stated interest rate	3.00%	4.30%

Repurchase agreements maturity date.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after the effects of master netting arrangements, of securities loaned to entities in exchange for collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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