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Collateralized Transactions - Schedule of Company Lines Available and Borrowings Outstanding (Parenthetical) (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	May 12, 2018	Mar. 28, 2018
Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 1,050,000,000	$ 950,000,000
Warehouse Line Due October 11, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date	Oct. 11, 2018	Oct. 11, 2018
Warehouse Line Due October 11, 2018 [Member] | Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 400,000,000	$ 300,000,000		$ 300,000
Warehouse Line Due October 11, 2018 [Member] | Scenario Forecast [Member] | Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available			$ 400,000
Warehouse Line Due September 25, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date	Sep. 25, 2018	Sep. 25, 2018
Warehouse Line Due September 25, 2018 [Member] | Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 200,000,000	$ 200,000,000
Warehouse Line Due June 20, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date	Jun. 20, 2018	Jun. 20, 2018
Warehouse Line Due June 20, 2018 [Member] | Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 450,000,000	$ 450,000,000

X

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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X

- Definition

The maximum amount of borrowing capacity under a line of credit that is available as of the balance sheet date for a specific purpose other than for financing goods acquired for inventory or imminent delivery to a customer.

+ References

No definition available.

+ Details

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X

- Definition

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+ References

No definition available.

+ Details

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