Warehouse Facilities Collateralized by U.S. Government Sponsored Enterprises - Schedule of Company Lines Available and Borrowings Outstanding (Detail) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018	Jan. 29, 2019
Securities Financing Transaction [Line Items]
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises		$ 680,860	$ 972,387
Committed Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		1,450,000	1,650,000
Uncommitted Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		600,000	325,000
Warehouse Facility Due June 17, 2020 [Member]
Securities Financing Transaction [Line Items]
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises		$ 188,507
Warehouse Facility Due June 17, 2020 [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR		1.15%
Warehouse Facility Due June 17, 2020 [Member] | Committed Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		$ 450,000
Warehouse Facility Due June 17, 2020 [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR	[1]	1.10%
Warehouse Facility Due June 17, 2020 [Member] | Uncommitted Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	[1]	$ 200,000
Warehouse Facility Due September 25, 2020 [Member]
Securities Financing Transaction [Line Items]
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises		$ 183,694
Warehouse Facility Due September 25, 2020 [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR		1.15%
Warehouse Facility Due September 25, 2020 [Member] | Committed Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		$ 200,000
Warehouse Facility Due October 9, 2020 [Member]
Securities Financing Transaction [Line Items]
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises	[2]	$ 145,159
Warehouse Facility Due October 9, 2020 [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR	[2]	1.15%
Warehouse Facility Due October 9, 2020 [Member] | Committed Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	[2]	$ 800,000
Fannie Mae Repurchase Agreement, Open Maturity [Member]
Securities Financing Transaction [Line Items]
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises		$ 163,500	$ 29,499
Fannie Mae Repurchase Agreement, Open Maturity [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR		1.05%	1.15%
Fannie Mae Repurchase Agreement, Open Maturity [Member] | Uncommitted Lines [Member]
Securities Financing Transaction [Line Items]
Lines available		$ 400,000	$ 325,000
Warehouse Facility Due June 19, 2019 [Member]
Securities Financing Transaction [Line Items]
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises			$ 413,063
Warehouse Facility Due June 19, 2019 [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR			1.20%
Warehouse Facility Due June 19, 2019 [Member] | Committed Lines [Member]
Securities Financing Transaction [Line Items]
Lines available			$ 450,000
Warehouse Facility Due September 25, 2019 [Member]
Securities Financing Transaction [Line Items]
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises			$ 113,452
Warehouse Facility Due September 25, 2019 [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR			1.20%
Warehouse Facility Due September 25, 2019 [Member] | Committed Lines [Member]
Securities Financing Transaction [Line Items]
Lines available			$ 200,000
Warehouse Facility Due October 10, 2019 [Member]
Securities Financing Transaction [Line Items]
Lines available				$ 1,000,000
Warehouse facilities collateralized by U.S. Government Sponsored Enterprises	[3]		$ 416,373
Warehouse Facility Due October 10, 2019 [Member] | One Month LIBOR [Member]
Securities Financing Transaction [Line Items]
Stated Spread to One Month LIBOR	[3]		1.20%
Warehouse Facility Due October 10, 2019 [Member] | Committed Lines [Member]
Securities Financing Transaction [Line Items]
Lines available	[3]		$ 1,000,000

[1] Effective October 2, 2019, the warehouse line was increased to $300.0 million

[2] On September 5, 2019, the warehouse facility was renewed, the notional amount was increased to $400.0 million, and the maturity date was extended to October 9, 2020. As of September 30, 2019, the warehouse line had a temporary increase of $400.0 million which expired on October 28, 2019.

[3] The warehouse facility was temporarily increased by $700.0 million to $1.0 billion for the period of November 30, 2018 to January 29, 2019. On January 29, 2019, the temporary increase was decreased by $400 million to $300 million for the period January 29, 2019 to April 1, 2019.