Debt - USD ($)				3 Months Ended						9 Months Ended
		Mar. 07, 2019	Aug. 06, 2018	Sep. 30, 2020		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019	Dec. 18, 2019
Debt Instrument [Line Items]
Maximum hedging reserve				$ 1,000,000.0						$ 1,000,000.0
Unamortized Debt Issuance Costs				(25,000,000.0)				$ (29,100,000)		$ (25,000,000.0)
Percentage of collections on contributed loans										6.00%
Monthly servicing fee per financing										6.00%
Mortgage note	[1]			10,700,000				11,300,000		$ 10,700,000
Repayment of senior notes							$ 148,200,000			401,800,000		$ 0
Senior notes				790,100,000				1,187,800,000		790,100,000
Proceeds from issuance of senior notes										0		400,000,000.0
Loss on extinguishment of debt				0		$ 7,400,000		1,800,000	$ 0	7,400,000		$ 0
Debt covenants				1						1
Unamortized Discount				0				(800,000)		0
Carrying Amount				4,619,300,000				4,538,800,000		4,619,300,000
Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Costs				(9,900,000)				(13,200,000)		(9,900,000)
Unamortized Discount				0				(800,000)		0
Carrying Amount				790,100,000				1,187,800,000		790,100,000
Mortgages [Member]
Debt Instrument [Line Items]
Debt facility financing amount			$ 12,000,000.0	12,000,000.0	[2]					12,000,000.0	[2]
Unamortized Debt Issuance Costs				0				0		0
Close date, secured financings			Aug. 06, 2018
Mortgage note				$ 10,700,000				$ 11,300,000		$ 10,700,000
Debt maturity date			Aug. 06, 2023							Aug. 06, 2023	[2]
Senior notes yield to maturity				1.66%				3.21%		1.66%
Unamortized Discount				$ 0				$ 0		$ 0
Carrying Amount				10,700,000				11,300,000		10,700,000
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility				340,000,000.0						$ 340,000,000.0
Percentage of net book value of loans										80.00%
Unamortized Debt Issuance Costs	[3]			0				0		$ 0
Unamortized Discount	[3]			0				0		0
Carrying Amount	[3]			125,700,000				$ 0		$ 125,700,000
Warehouse Facilities [Member]
Debt Instrument [Line Items]
Percentage of net book value of loans										80.00%
Number of warehouse facilities										6
Debt facility financing amount				1,225,000,000.0						$ 1,225,000,000.0
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2],[4]			$ 400,000,000.0						$ 400,000,000.0
Debt maturity date	[2],[5]									Jul. 12, 2022
Senior notes yield to maturity				1.91%				0.00%		1.91%
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Debt facility financing amount				$ 300,000,000.0	[2],[4]			$ 300,000,000.0		$ 300,000,000.0	[2],[4]
Debt maturity date	[2],[5]									Jul. 26, 2022
Senior notes yield to maturity				0.00%				0.00%		0.00%
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2],[4]			$ 100,000,000.0						$ 100,000,000.0
Debt maturity date	[2],[6]									Aug. 17, 2021
Senior notes yield to maturity				0.00%				0.00%		0.00%
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 75,000,000.0						$ 75,000,000.0
Debt maturity date	[2],[5]									Sep. 30, 2022
Senior notes yield to maturity				2.16%				0.00%		2.16%
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2],[4]			$ 150,000,000.0						$ 150,000,000.0
Debt maturity date	[2],[7]									Dec. 16, 2021
Senior notes yield to maturity				2.39%				0.00%		2.39%
Term ABS 2016-3 [Member]
Debt Instrument [Line Items]
Senior notes yield to maturity				0.00%				3.60%		0.00%
Term ABS 2017-1 [Member]
Debt Instrument [Line Items]
Senior notes yield to maturity				0.00%				3.19%		0.00%
Term ABS 2017-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 450,000,000.0						$ 450,000,000.0
Debt financing close date										Jun. 29, 2017
Debt maturity date	[2],[5]									Jun. 17, 2019
Senior notes yield to maturity				3.24%				2.83%		3.24%
Net Book Value Of Loans Pledged As Collateral				$ 563,200,000						$ 563,200,000
Term ABS 2017-3 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 350,000,000.0						$ 350,000,000.0
Debt financing close date										Oct. 26, 2017
Debt maturity date	[2],[5]									Oct. 15, 2019
Senior notes yield to maturity				3.31%				2.91%		3.31%
Net Book Value Of Loans Pledged As Collateral				$ 437,600,000						$ 437,600,000
Term ABS 2018-1 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 500,000,000.0						$ 500,000,000.0
Debt financing close date										Feb. 22, 2018
Debt maturity date	[2],[5]									Feb. 17, 2020
Senior notes yield to maturity				3.43%				3.24%		3.43%
Net Book Value Of Loans Pledged As Collateral				$ 625,100,000						$ 625,100,000
Term ABS 2018-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 450,000,000.0						$ 450,000,000.0
Debt financing close date										May 24, 2018
Debt maturity date	[2],[5]									May 15, 2020
Senior notes yield to maturity				3.76%				3.68%		3.76%
Net Book Value Of Loans Pledged As Collateral				$ 562,600,000						$ 562,600,000
Term ABS 2018-3 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 398,300,000						$ 398,300,000
Debt financing close date										Aug. 23, 2018
Debt maturity date	[2],[5]									Aug. 17, 2020
Senior notes yield to maturity				3.73%				3.72%		3.73%
Net Book Value Of Loans Pledged As Collateral				$ 500,100,000						$ 500,100,000
Term ABS 2019-1 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 402,500,000						$ 402,500,000
Debt financing close date										Feb. 21, 2019
Debt maturity date	[2],[5]									Feb. 15, 2021
Senior notes yield to maturity				3.53%				3.53%		3.53%
Net Book Value Of Loans Pledged As Collateral				$ 503,100,000						$ 503,100,000
Term ABS 2019-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 500,000,000.0						$ 500,000,000.0
Debt financing close date										Aug. 28, 2019
Debt maturity date	[2],[8]									Aug. 15, 2022
Senior notes yield to maturity				3.13%				3.13%		3.13%
Net Book Value Of Loans Pledged As Collateral				$ 625,100,000						$ 625,100,000
Term ABS 2019-3 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 351,700,000						$ 351,700,000
Debt financing close date										Nov. 21, 2019
Debt maturity date	[2],[5]									Nov. 15, 2021
Senior notes yield to maturity				2.56%				2.56%		2.56%
Net Book Value Of Loans Pledged As Collateral				$ 439,600,000						$ 439,600,000
Term ABS 2020-1 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[2]			$ 500,000,000.0						$ 500,000,000.0
Debt financing close date										Feb. 20, 2020
Debt maturity date	[2],[5]									Feb. 15, 2022
Senior notes yield to maturity				2.18%						2.18%
Net Book Value Of Loans Pledged As Collateral				$ 625,100,000						$ 625,100,000
Term ABS 2020-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount				$ 481,800,000						$ 481,800,000
Debt financing close date										Jul. 23, 2020
Debt maturity date										Jul. 15, 2022
Senior notes yield to maturity				1.65%						1.65%
Net Book Value Of Loans Pledged As Collateral				$ 602,300,000						$ 602,300,000
2021 Senior Notes [Member]
Debt Instrument [Line Items]
Debt facility financing amount													$ 300,000,000.0
Repayment of senior notes						$ 151,800,000
Senior notes				$ 0				$ 151,800,000		$ 0
Senior notes yield to maturity				0.00%				6.125%		0.00%			6.125%
2024 Senior Notes [Member]
Debt Instrument [Line Items]
Debt facility financing amount				$ 400,000,000.0						$ 400,000,000.0
Debt maturity date										Dec. 31, 2024
Senior notes				$ 400,000,000.0				$ 400,000,000.0		$ 400,000,000.0			$ 400,000,000.0
Senior notes yield to maturity				5.125%				5.125%		5.125%			5.125%
2026 Senior Notes [Member]
Debt Instrument [Line Items]
Debt facility financing amount		$ 400,000,000.0		$ 400,000,000.0						$ 400,000,000.0
Close date, secured financings		Mar. 07, 2019
Senior notes stated interest rate		6.625%
Debt maturity date		Mar. 15, 2026								Mar. 15, 2026
Senior notes				$ 400,000,000.0				$ 400,000,000.0		$ 400,000,000.0
Senior notes yield to maturity				6.625%				6.625%		6.625%
LIBOR [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	[2],[4]									150.00%
LIBOR [Member] | Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate										187.50%
LIBOR [Member] | Warehouse Facility II [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	[2],[4]									175.00%
LIBOR [Member] | Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	[2],[4]									200.00%
LIBOR [Member] | Warehouse Facility V [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	[2],[4]									190.00%
LIBOR [Member] | Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	[2]									200.00%
Prime Rate [Member] | Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate										87.50%

[1] Measured at amortized cost with fair value disclosed.

[2] Financing made available only to a specified subsidiary of the Company.

[3] Excludes deferred debt issuance costs of $2.2 million and $3.2 million as of September 30, 2020 and December 31, 2019, respectively, which are included in other assets.

[4] Interest rate cap agreements are in place to limit the exposure to increasing interest rates

[5] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date based on the cash flows of the pledged assets.

[6] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on August 17, 2023 will be due on that date.

[7] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on December 16, 2023 will be due on that date.

[8] Represents the revolving maturity date. The Company has the option to redeem and retire the indebtedness after the revolving maturity date. If we do not elect this option, the outstanding balance will amortize based on the cash flows of the pledged assets.