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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 |  |

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- 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 + Details
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