Warehouse Receivables & Warehouse Lines of Credit - Summary of Warehouse Lines of Credit in Place (Details) - USD ($)				12 Months Ended
	Jan. 18, 2021	Sep. 25, 2020	Jul. 24, 2020	Dec. 31, 2020	Dec. 01, 2020	Oct. 19, 2020	Sep. 22, 2020	Sep. 21, 2020	Aug. 04, 2020	Jul. 01, 2020	Jun. 10, 2020	Dec. 31, 2019	Jun. 28, 2019
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Carrying Value				$ 1,383,964,000								$ 977,175,000
Warehouse Agreement Borrowings
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Maximum Facility Size				3,500,000,000								3,400,000,000
Carrying Value				$ 1,383,964,000								977,175,000
Warehouse Agreement Borrowings | JP Morgan | Pricing at daily one-month LIBOR plus 1.30%, maturing October 18, 2021
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity				Oct. 18, 2021
Line of credit over LIBOR rate				1.60%
Maximum Facility Size				$ 1,585,000,000	$ 2,085,000,000.0	$ 1,585,000,000.0						985,000,000
Carrying Value				$ 561,726,000								267,075,000
Warehouse Agreement Borrowings | JP Morgan | Pricing at daily one-month LIBOR plus 1.30%, maturing October 18, 2021 | Subsequent Event
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Line of credit over LIBOR rate	1.60%
Maximum Facility Size	$ 985,000,000.0
Warehouse Agreement Borrowings | JP Morgan | Pricing at daily one-month LIBOR plus 2.75%, maturing October 18, 2021
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity				Oct. 18, 2021
Line of credit over LIBOR rate				2.75%
Maximum Facility Size				$ 15,000,000								15,000,000
Warehouse Agreement Borrowings | Capital One | Warehouse Line Of Credit Three
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Maximum Facility Size				0								200,000,000
Carrying Value				$ 0								39,538,000
Warehouse Agreement Borrowings | Fannie Mae ASAP Program
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity				Cancelableanytime
Warehouse Agreement Borrowings | Fannie Mae ASAP Program | Pricing at daily one-month LIBOR plus 1.45%, with a LIBOR floor of 0.25%, Cancelable anytime
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Line of credit over LIBOR rate				1.45%
Line of credit, LIBOR floor rate		0.25%		0.25%
Maximum Facility Size				$ 450,000,000								450,000,000
Carrying Value				$ 132,692,000								360,784,000
Interest rate increase (decrease)		1000.00%
Warehouse Agreement Borrowings | T D Bank | Pricing 2-business day prior LIBOR plus 1.15%, maturing June 30, 2021
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity				Jun. 30, 2021
Line of credit over LIBOR rate				1.15%
Maximum Facility Size				$ 800,000,000						$ 400,000,000.0		800,000,000
Carrying Value				$ 401,849,000								92,266,000
Line of credit utilized								$ 400,000,000.0
Warehouse Agreement Borrowings | T D Bank | Warehouse Line Of Credit Five, temporary, uncommitted
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Maximum Facility Size										$ 400,000,000.0
Warehouse Agreement Borrowings | Bank Of America | Pricing at (i) daily floating rate LIBOR plus 1.40%, with a LIBOR floor of 0.25% on the general facility (ii) daily floating rate LIBOR pluc 1.75% on sublimit borrowing, maturing May 26, 2021
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity				May 26, 2021
Maximum Facility Size											$ 350,000,000	350,000,000
Carrying Value				$ 175,862,000								189,465,000
Warehouse Agreement Borrowings | Bank Of America | Pricing at (i) daily floating rate LIBOR plus 1.40%, with a LIBOR floor of 0.25% on the general facility (ii) daily floating rate LIBOR pluc 1.75% on sublimit borrowing, maturing May 26, 2021 | General Facility
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Line of credit over LIBOR rate			1.40%
Line of credit, LIBOR floor rate			0.25%
Warehouse Agreement Borrowings | Bank Of America | Pricing at (i) daily floating rate LIBOR plus 1.40%, with a LIBOR floor of 0.25% on the general facility (ii) daily floating rate LIBOR pluc 1.75% on sublimit borrowing, maturing May 26, 2021 | Sublimit Borrowing Agreement
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Line of credit over LIBOR rate			1.75%
Maximum Facility Size											$ 100,000,000.0
Warehouse Agreement Borrowings | Bank Of America | Warehouse Line Of Credit Seven
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Maximum Facility Size				0								250,000,000
Carrying Value				$ 0								17,457,000
Warehouse Agreement Borrowings | MUFG Union Bank, N.A. (Union Bank) | Pricing at daily floating rate LIBOR plus 1.50%, maturing June 28, 2021
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Current Maturity				Jun. 28, 2021
Line of credit over LIBOR rate				1.50%
Line of credit, LIBOR floor rate				0.25%
Maximum Facility Size				$ 300,000,000								350,000,000	$ 200,000,000.0
Carrying Value				$ 111,835,000								$ 10,590,000
Additional line of credit									$ 100,000,000.0				$ 150,000,000.0
Warehouse Agreement Borrowings | MUFG Union Bank, N.A. (Union Bank) | Pricing at daily floating rate LIBOR plus 1.50%, maturing June 28, 2021 | Maximum
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]
Line of credit utilized							$ 100,000,000.0