Warehouse Receivables & Warehouse Lines of Credit - Summary of Warehouse Lines of Credit in Place (Parenthetical) (Detail) - Warehouse Agreement Borrowings [Member] - USD ($)				1 Months Ended	9 Months Ended	12 Months Ended
	Oct. 21, 2019	Aug. 01, 2019	Jul. 01, 2019	Jul. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 09, 2019	Sep. 08, 2019	Jun. 28, 2019	Apr. 01, 2019	Feb. 27, 2019	Jan. 31, 2019
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Maximum Facility Size					$ 3,400,000,000	$ 2,600,000,000
JP Morgan [Member] \| Pricing at daily one-month LIBOR plus 1.30%, maturing October 21, 2019 [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity					Oct. 21, 2019
Maximum Facility Size					$ 985,000,000	$ 985,000,000
Line of credit over LIBOR rate					1.30%	1.30%
JP Morgan [Member] \| Pricing at daily one-month LIBOR plus 1.30%, maturing October 21, 2019 [Member] \| Subsequent Event [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity	Oct. 19, 2020
JP Morgan [Member] \| Pricing at daily one-month LIBOR plus 2.75%, maturing October 21, 2019 [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity					Oct. 21, 2019
Maximum Facility Size					$ 15,000,000	$ 15,000,000
Line of credit over LIBOR rate					2.75%	2.75%
JP Morgan [Member] \| Pricing at daily one-month LIBOR plus 2.75%, maturing October 21, 2019 [Member] \| Subsequent Event [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity	Oct. 19, 2020
Capital One [Member] \| Pricing at daily one-month LIBOR plus 1.25%, maturing July 27, 2020 [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity				Jul. 27, 2020
Maximum Facility Size				$ 200,000,000.0		$ 325,000,000.0				$ 200,000,000.0	$ 700,000,000.0	$ 200,000,000.0
Line of credit over LIBOR rate				1.25%
Fannie Mae ASAP Program [Member] \| Pricing at daily one-month LIBOR plus 1.35%, with a LIBOR floor of 0.35%, Cancelable anytime [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Maximum Facility Size					$ 450,000,000	$ 450,000,000	$ 600,000,000.0	$ 450,000,000.0
Line of credit over LIBOR rate					1.35%	1.35%
TD Bank [Member] \| Pricing at daily one-month LIBOR plus 1.15%, maturing June 30, 2020 [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity		Feb. 01, 2020	Jun. 30, 2020
Maximum Facility Size		$ 800,000,000.0
Line of credit over LIBOR rate			1.15%
MUFG Union Bank, N.A. (Union Bank) [Member] \| Pricing at daily one-month LIBOR plus 1.20%, maturing June 28, 2020 [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity					Jun. 28, 2020
Maximum Facility Size					$ 350,000,000				$ 200,000,000.0
Line of credit over LIBOR rate					1.20%	1.20%
MUFG Union Bank, N.A. (Union Bank) [Member] \| Pricing at daily one-month LIBOR plus 1.20%, maturing June 28, 2020 [Member] \| Maximum [Member]
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Additional line of credit									$ 150,000,000.0