Loans Receivable (Summary Of Changes In Loans Receivable) (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period			$ 5,049.0		$ 4,207.0		$ 3,345.1
New Consumer Loan assignments	[1]		3,595.8		2,873.1		2,635.5
Principal collected on Loans receivable			(2,576.7)		(2,189.5)		(1,955.8)
Accelerated Dealer Holdback payments			(52.6)		(47.1)		(53.6)
Dealer Holdback payments			128.9		131.6		142.0
Transfers	[2]		0.0		0.0		0.0
Write-offs			(28.6)		(22.1)		(14.8)
Recoveries	[3]		4.2	[4]	1.8	[5]	1.4	[5]
Balance, end of period		$ 5,049.0	6,225.2		5,049.0		4,207.0
Dealer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period			3,518.1		3,209.0		2,823.4
New Consumer Loan assignments	[1]		2,414.8		1,968.3		1,881.3
Principal collected on Loans receivable			(1,873.0)		(1,729.9)		(1,668.1)
Accelerated Dealer Holdback payments			(52.6)		(47.1)		(53.6)
Dealer Holdback payments			128.9		131.6		142.0
Transfers	[2]	(89.0)	(78.2)		(93.1)		(10.1)
Write-offs			(25.2)		(16.4)		(14.4)
Recoveries	[3]		3.0	[4]	1.5	[4]	1.3	[5]
Balance, end of period		3,518.1	4,141.0		3,518.1		3,209.0
Purchased Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period			1,530.9		998.0		521.7
New Consumer Loan assignments	[1]		1,181.0		904.8		754.2
Principal collected on Loans receivable			(703.7)		(459.6)		(287.7)
Accelerated Dealer Holdback payments			0.0		0.0		0.0
Dealer Holdback payments			0.0		0.0		0.0
Transfers	[2]		78.2		93.1		10.1
Write-offs			(3.4)		(5.7)		(0.4)
Recoveries	[3]		1.2	[4]	0.3	[5]	0.1	[5]
Balance, end of period		$ 1,530.9	$ 2,084.2		$ 1,530.9		$ 998.0

[1] The Dealer Loans amount represents advances paid to Dealers on Consumer Loans assigned under our Portfolio Program. The Purchased Loans amount represents one-time payments made to Dealers to purchase Consumer Loans assigned under our Purchase Program.

[2] 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 to Purchased Loans in the period this forfeiture occurs.

[3] Represents collections received on previously written off Loans.

[4] Represents collections received on previously written off Loans.

[5] 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 to Purchased Loans in the period this forfeiture occurs. Beginning in the fourth quarter of 2017, we also transfer the related allowance for credit losses balance to Purchased Loans in the period this forfeiture occurs.