Loans Receivable (Summary of Changes in Loans Receivable) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		$ 5,780.1	$ 4,637.7	$ 5,049.0	$ 4,207.0
New Consumer Loan assignments	[1]	831.0	690.6	2,787.2	2,153.9
Principal collected on Loans receivable		(647.3)	(541.1)	(1,949.0)	(1,657.4)
Accelerated Dealer Holdback payments		(11.6)	(11.9)	(41.0)	(35.5)
Dealer Holdback payments		31.9	32.1	98.0	100.8
Write-offs		(2.7)	(4.1)	(24.5)	(13.5)
Recoveries	[2],[3]	0.6	0.5	3.5	1.3
Balance, end of period		6,005.2	4,827.6	6,005.2	4,827.6
Dealer Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		3,918.4	3,385.3	3,518.1	3,209.0
New Consumer Loan assignments	[1]	556.7	474.0	1,870.2	1,467.2
Principal collected on Loans receivable		(466.0)	(424.6)	(1,425.2)	(1,323.8)
Accelerated Dealer Holdback payments		(11.6)	(11.9)	(41.0)	(35.5)
Dealer Holdback payments		31.9	32.1	98.0	100.8
Transfers	[4]	(20.2)	(1.9)	(53.3)	4.1
Write-offs		(2.5)	(2.9)	(21.3)	(11.4)
Recoveries	[2],[3]	0.3	0.4	2.7	1.1
Balance, end of period		4,030.2	3,474.3	4,030.2	3,474.3
Purchased Loans [Member]
Loans and Leases Receivable, Net [Roll Forward]
Balance, beginning of period		1,861.7	1,252.4	1,530.9	998.0
New Consumer Loan assignments	[1]	274.3	216.6	917.0	686.7
Principal collected on Loans receivable		(181.3)	(116.5)	(523.8)	(333.6)
Transfers	[4]	20.2	1.9	53.3	4.1
Write-offs		(0.2)	(1.2)	(3.2)	(2.1)
Recoveries	[2],[3]	0.3	0.1	0.8	0.2
Balance, end of period		$ 1,975.0	$ 1,353.3	$ 1,975.0	$ 1,353.3

[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] Represents collections received on previously written off Loans.

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

[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 to Purchased Loans in the period this forfeiture occurs.