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Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 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	$ 55.1	$ 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	$ 337.3	$ 255.1