Loans Receivable Loans Receivable (Summary of Changes in Expected Future Net Cash Flows) (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2023		Sep. 30, 2022	Sep. 30, 2023		Sep. 30, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period		$ 9,676.3		$ 9,042.2	$ 9,033.4		$ 8,948.3
New Consumer Loan Assignments	[1],[2]	1,442.7		1,279.8	4,475.8		3,895.3
Realized Net Cash Flows		(1,112.4)	[3]	(1,179.5)	(3,431.2)	[3]	(3,773.8)	[3]
Forecast changes		(69.4)		(85.4)	(149.3)		(18.6)
Transfers	[4]	2.0		1.7	10.5		7.6
Balance, end of period		9,939.2		9,058.8	9,939.2		9,058.8
Dealer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period		6,166.4		5,468.6	5,637.9		5,249.7
New Consumer Loan Assignments	[1],[2]	1,035.1		905.6	3,118.1		2,736.4
Realized Net Cash Flows		(705.7)	[3]	(730.6)	(2,164.3)	[3]	(2,314.1)	[3]
Forecast changes		(40.3)		(37.3)	(89.3)		(17.4)
Transfers	[4]	(26.5)		(15.2)	(73.4)		(63.5)
Balance, end of period		6,429.0		5,591.1	6,429.0		5,591.1
Purchased Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period		3,509.9		3,573.6	3,395.5		3,698.6
New Consumer Loan Assignments	[1],[2]	407.6		374.2	1,357.7		1,158.9
Realized Net Cash Flows		(406.7)	[3]	(448.9)	(1,266.9)	[3]	(1,459.7)	[3]
Forecast changes		(29.1)		(48.1)	(60.0)		(1.2)
Transfers	[4]	28.5		16.9	83.9		71.1
Balance, end of period		$ 3,510.2		$ 3,467.7	$ 3,510.2		$ 3,467.7

[1] The Dealer Loans amount represents repayments that we expected to collect at the time of assignment on Consumer Loans assigned under our Portfolio Program, less the related Dealer Holdback payments that we expected to make. The Purchased Loans amount represents repayments that we expected to collect at the time of assignment on Consumer Loans assigned under our Purchase Program.

[2] The Dealer Loans amount represents repayments that we expected to collect at the time of assignment on Consumer Loans assigned under our Portfolio Program, less the related Dealer Holdback payments that we expected to make. The Purchased Loans amount represents repayments that we expected to collect at the time of assignment on Consumer Loans assigned under our Purchase Program. The Loan amounts also represent the fair value at the time of assignment.

[3] The Dealer Loans amount represents repayments that we collected on Consumer Loans assigned under our Portfolio Program, less the Dealer Holdback and Accelerated Dealer Holdback payments that we made. Purchased Loans amount represents repayments that we collected on Consumer Loans assigned under our Purchase Program.

[4] Under our Portfolio Program, certain events may result in Dealers forfeiting their rights to Dealer Holdback. We transfer the Dealer’s outstanding Dealer Loan balance, related allowance for credit losses balance, and related expected future net cash flows to Purchased Loans in the period this forfeiture occurs.