Loans Receivable (Summary Of Changes In Loans Receivable) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period		$ 4,207.0		$ 3,345.1		$ 2,719.8
New Consumer Loan assignments	[1]	2,873.1		2,635.5		2,167.0
Principal collected on Loans receivable		(2,189.5)		(1,955.8)		(1,739.6)
Accelerated Dealer Holdback payments		(47.1)		(53.6)		(52.6)
Dealer Holdback payments		131.6		142.0		150.1
Transfers	[2]	0.0		0.0		0.0
Write-offs		(22.1)		(14.8)		(6.6)
Recoveries	[4]	1.8	[3]	1.4	[5]	1.8	[5]
Balance, end of period		5,049.0		4,207.0		3,345.1
Dealer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period		3,209.0		2,823.4		2,389.8
New Consumer Loan assignments	[1]	1,968.3		1,881.3		1,795.1
Principal collected on Loans receivable		(1,729.9)		(1,668.1)		(1,548.8)
Accelerated Dealer Holdback payments		(47.1)		(53.6)		(52.6)
Dealer Holdback payments		131.6		142.0		150.1
Transfers	[2]	(93.1)		(10.1)		(10.6)
Write-offs		(16.4)		(14.4)		(6.4)
Recoveries	[4]	1.5	[3]	1.3	[3]	1.6	[5]
Balance, end of period		3,518.1		3,209.0		2,823.4
Purchased Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period		998.0		521.7		330.0
New Consumer Loan assignments	[1]	904.8		754.2		371.9
Principal collected on Loans receivable		(459.6)		(287.7)		(190.8)
Accelerated Dealer Holdback payments		0.0		0.0		0.0
Dealer Holdback payments		0.0		0.0		0.0
Transfers	[2]	93.1		10.1		10.6
Write-offs		(5.7)		(0.4)		(0.2)
Recoveries	[4]	0.3	[3]	0.1	[5]	0.2	[5]
Balance, end of period		$ 1,530.9		$ 998.0		$ 521.7

[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.