Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) portfolio_segment segment	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of reportable segments | segment	1
Uninsured cash and cash equivalents	$ 10.8	$ 7.8
Number of portfolio segments | portfolio_segment	2
Period after Consumer Loan assignment that Dealer Loans written off	120 months
Period after month of purchase that Purchased Loans written off	120 months
Restricted Cash and Cash Equivalents [Member]
Summary Of Significant Accounting Policies [Line Items]
Uninsured cash and cash equivalents	$ 367.9	$ 255.1