Loans Receivable (Summary of Changes in Loans Receivable and Allowance for Credit Losses) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		$ (6,685.2)	$ (5,763.3)
Adoption of CECL	[1]	0.0
Finance Charges Revenue		(361.9)	(321.9)
Provision for credit losses		(354.7)	(14.5)
New Consumer Loan Assignments	[2]	1,055.3	1,105.4
Proceeds from Collection of Loans Receivable	[3]	(1,179.4)	(1,081.5)
Accelerated Payments of Dealer Holdback		11.4	12.3
Payments of Dealer Holdback		36.6	34.6
Loans And Leases Receivable Transfers	[4]	0.0	0.0
Allowance for Loan and Lease Losses, Write-offs		0.0	0.0
Recoveries	[5]	0.0	0.0
Deferral of Loan Origination Costs		2.2	2.2
Balance, end of period		(6,618.5)	(6,143.7)
Dealer Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(4,195.3)	(3,762.9)
Adoption of CECL	[1]	0.0
Finance Charges Revenue		(228.1)	(213.5)
Provision for credit losses		(168.1)	(11.6)
New Consumer Loan Assignments	[2]	638.1	719.0
Proceeds from Collection of Loans Receivable	[3]	(776.6)	(747.0)
Accelerated Payments of Dealer Holdback		11.4	12.3
Payments of Dealer Holdback		36.6	34.6
Loans And Leases Receivable Transfers	[4]	(21.6)	(21.5)
Allowance for Loan and Lease Losses, Write-offs		0.0	0.0
Recoveries	[5]	0.0	0.0
Deferral of Loan Origination Costs		2.2	2.2
Balance, end of period		(4,145.4)	(3,964.4)
Purchased Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(2,489.9)	(2,000.4)
Adoption of CECL	[1]	0.0
Finance Charges Revenue		(133.8)	(108.4)
Provision for credit losses		(186.6)	(2.9)
New Consumer Loan Assignments	[2]	417.2	386.4
Proceeds from Collection of Loans Receivable	[3]	(402.8)	(334.5)
Accelerated Payments of Dealer Holdback		0.0	0.0
Payments of Dealer Holdback		0.0	0.0
Loans And Leases Receivable Transfers	[4]	(21.6)	(21.5)
Allowance for Loan and Lease Losses, Write-offs		0.0	0.0
Recoveries	[5]	0.0	0.0
Deferral of Loan Origination Costs		0.0	0.0
Balance, end of period		(2,473.1)	(2,179.3)
Loans Receivable [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(7,221.2)	(6,225.2)
Adoption of CECL	[1]	2,463.6
Finance Charges Revenue		(533.8)	(321.9)
Provision for credit losses		0.0	0.0
New Consumer Loan Assignments	[2]	1,055.3	1,105.4
Proceeds from Collection of Loans Receivable	[3]	(1,179.4)	(1,081.5)
Accelerated Payments of Dealer Holdback		11.4	12.3
Payments of Dealer Holdback		36.6	34.6
Loans And Leases Receivable Transfers	[4]	0.0	0.0
Allowance for Loan and Lease Losses, Write-offs		(286.3)	(2.9)
Recoveries	[5]	(0.6)	(0.7)
Deferral of Loan Origination Costs		2.2	2.2
Balance, end of period		(9,859.0)	(6,617.9)
Loans Receivable [Member] | Dealer Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(4,623.3)	(4,141.0)
Adoption of CECL	[1]	940.2
Finance Charges Revenue		(311.8)	(213.5)
Provision for credit losses		0.0	0.0
New Consumer Loan Assignments	[2]	638.1	719.0
Proceeds from Collection of Loans Receivable	[3]	(776.6)	(747.0)
Accelerated Payments of Dealer Holdback		11.4	12.3
Payments of Dealer Holdback		36.6	34.6
Loans And Leases Receivable Transfers	[4]	(31.3)	(25.7)
Allowance for Loan and Lease Losses, Write-offs		(78.3)	(2.8)
Recoveries	[5]	(0.3)	(0.4)
Deferral of Loan Origination Costs		2.2	2.2
Balance, end of period		(5,677.7)	(4,347.5)
Loans Receivable [Member] | Purchased Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(2,597.9)	(2,084.2)
Adoption of CECL	[1]	1,523.4
Finance Charges Revenue		(222.0)	(108.4)
Provision for credit losses		0.0	0.0
New Consumer Loan Assignments	[2]	417.2	386.4
Proceeds from Collection of Loans Receivable	[3]	(402.8)	(334.5)
Accelerated Payments of Dealer Holdback		0.0	0.0
Payments of Dealer Holdback		0.0	0.0
Loans And Leases Receivable Transfers	[4]	(31.3)	(25.7)
Allowance for Loan and Lease Losses, Write-offs		(208.0)	(0.1)
Recoveries	[5]	(0.3)	(0.3)
Deferral of Loan Origination Costs		0.0	0.0
Balance, end of period		(4,181.3)	(2,270.4)
Allowance for Credit Losses [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(536.0)	(461.9)
Adoption of CECL	[1]	2,463.6
Finance Charges Revenue		(171.9)	0.0
Provision for credit losses		(354.7)	(14.5)
New Consumer Loan Assignments	[2]	0.0	0.0
Proceeds from Collection of Loans Receivable	[3]	0.0	0.0
Accelerated Payments of Dealer Holdback		0.0	0.0
Payments of Dealer Holdback		0.0	0.0
Loans And Leases Receivable Transfers	[4]	0.0	0.0
Allowance for Loan and Lease Losses, Write-offs		(286.3)	(2.9)
Recoveries	[5]	(0.6)	(0.7)
Deferral of Loan Origination Costs		0.0	0.0
Balance, end of period		(3,240.5)	(474.2)
Allowance for Credit Losses [Member] | Dealer Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(428.0)	(378.1)
Adoption of CECL	[1]	940.2
Finance Charges Revenue		(83.7)	0.0
Provision for credit losses		(168.1)	(11.6)
New Consumer Loan Assignments	[2]	0.0	0.0
Proceeds from Collection of Loans Receivable	[3]	0.0	0.0
Accelerated Payments of Dealer Holdback		0.0	0.0
Payments of Dealer Holdback		0.0	0.0
Loans And Leases Receivable Transfers	[4]	(9.7)	(4.2)
Allowance for Loan and Lease Losses, Write-offs		(78.3)	(2.8)
Recoveries	[5]	(0.3)	(0.4)
Deferral of Loan Origination Costs		0.0	0.0
Balance, end of period		(1,532.3)	(383.1)
Allowance for Credit Losses [Member] | Purchased Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		(108.0)	(83.8)
Adoption of CECL	[1]	1,523.4
Finance Charges Revenue		(88.2)	0.0
Provision for credit losses		(186.6)	(2.9)
New Consumer Loan Assignments	[2]	0.0	0.0
Proceeds from Collection of Loans Receivable	[3]	0.0	0.0
Accelerated Payments of Dealer Holdback		0.0	0.0
Payments of Dealer Holdback		0.0	0.0
Loans And Leases Receivable Transfers	[4]	(9.7)	(4.2)
Allowance for Loan and Lease Losses, Write-offs		(208.0)	(0.1)
Recoveries	[5]	(0.3)	(0.3)
Deferral of Loan Origination Costs		0.0	0.0
Balance, end of period		$ (1,708.2)	$ (91.1)

[1] Represents the gross-up of Loans receivable and allowance for credit losses on January 1, 2020 upon the adoption of CECL for the present value of the difference between contractual future net cash flows and expected future net cash flows discounted at the effective interest rate.

[2] 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.

[3] Represents repayments that we collected on Consumer Loans assigned under our programs.

[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 and related allowance for credit losses balance to Purchased Loans in the period this forfeiture occurs

[5] The Dealer Loans amount represents net cash flows received (collection less any related Dealer Holdback payments) on Dealer Loans that were previously written off in full. The Purchased Loans amount represents collections received on Purchased Loans that were previously written off in full.