Collateralized Transactions - Schedule of Company Lines Available and Borrowings Outstanding (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Borrowings outstanding	$ 1,131,792	$ 360,440
Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	1,650,000	950,000
Uncommited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	325,000	325,000
Warehouse Line Due June 19, 2019 [Member]
Securities Financing Transaction [Line Items]
Borrowings outstanding	$ 255,419
Rate Type	Variable
Warehouse Line Due June 19, 2019 [Member] \| One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate	1.30%
Warehouse Line Due June 19, 2019 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 450,000
Warehouse Line Due September 25, 2019 [Member]
Securities Financing Transaction [Line Items]
Borrowings outstanding	$ 107,986
Rate Type	Variable
Warehouse Line Due September 25, 2019 [Member] \| One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate	1.20%
Warehouse Line Due September 25, 2019 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 200,000
Warehouse Line Due October 10, 2019 [Member]
Securities Financing Transaction [Line Items]
Borrowings outstanding	$ 724,801
Rate Type	Variable
Warehouse Line Due October 10, 2019 [Member] \| One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate	1.20%
Warehouse Line Due October 10, 2019 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 1,000,000
Fannie Mae Repurchase Agreement, Open Maturity [Member]
Securities Financing Transaction [Line Items]
Borrowings outstanding	$ 43,586	$ 18,240
Rate Type	Variable	Variable
Fannie Mae Repurchase Agreement, Open Maturity [Member] \| One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate	1.20%	1.20%
Fannie Mae Repurchase Agreement, Open Maturity [Member] \| Uncommited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	$ 325,000	$ 325,000
Warehouse Line Due June 20, 2018 [Member]
Securities Financing Transaction [Line Items]
Borrowings outstanding		$ 60,715
Rate Type		Variable
Warehouse Line Due June 20, 2018 [Member] \| One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate		1.30%
Warehouse Line Due June 20, 2018 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		$ 450,000
Warehouse Line Due September 25, 2018 [Member]
Securities Financing Transaction [Line Items]
Borrowings outstanding		$ 107,383
Rate Type		Variable
Warehouse Line Due September 25, 2018 [Member] \| One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate		1.30%
Warehouse Line Due September 25, 2018 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		$ 200,000
Warehouse Line Due October 11, 2018 [Member]
Securities Financing Transaction [Line Items]
Borrowings outstanding		$ 174,102
Rate Type		Variable
Warehouse Line Due October 11, 2018 [Member] \| One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate		1.30%
Warehouse Line Due October 11, 2018 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		$ 300,000

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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- 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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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Carrying value as of the balance sheet date of the outstanding short-term borrowings under a revolving line of credit used by mortgage bankers to finance the origination or purchase of loans.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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