Related Party Transactions (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Affiliated Dealer Loan balances		$ 0.0	$ 1.4
Percentage of Affiliated Dealer Loan balances		0.00%
New Consumer Loan assignments		$ 1,968.3	1,881.3	$ 1,795.1
Accelerated Dealer Holdback payments		47.1	53.6	52.6
Dealer Holdback payments		$ 131.6	142.0	150.1
Affiliated Dealer Activity [Member]
Related Party Transaction [Line Items]
Dealer Loan revenue			$ 1.5	$ 2.5
Dealer Loan revenue, percentage			0.20%	0.40%
New Consumer Loan assignments	[1]		$ 8.9	$ 5.9
New Consumer Loan assignments, percentage			0.30%	0.30%
Accelerated Dealer Holdback payments			$ 0.2	$ 0.2
Accelerated Dealer Holdback payments, percentage			0.40%	0.40%
Dealer Holdback payments			$ 1.0	$ 1.4
Dealer Holdback payments, percentage			0.70%	0.90%

[1] Represents advances paid to Dealers on Consumer Loans assigned under our Portfolio Program and one-time payments made to Dealers to purchase Consumer Loans assigned under our Purchase Program.