Consolidated Statements Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows From Operating Activities:
Net income	$ 574.0	$ 470.2	$ 332.8
Adjustments to reconcile cash provided by operating activities:
Provision for credit losses	56.9	129.3	90.2
Depreciation	5.4	6.0	6.1
Amortization	14.1	10.4	9.2
Increase (Decrease) in Deferred Income Taxes	49.3	(85.6)	24.2
Stock-based compensation	10.3	15.4	7.4
Other Operating Activities, Cash Flow Statement	(0.2)	0.1	0.1
Change in operating assets and liabilities:
Increase in accounts payable and accrued liabilities	41.6	10.3	11.8
Increase in income taxes receivable	(5.7)	0.1	7.7
Increase (decrease) in income taxes payable	(37.4)	16.3	23.6
Increase in other assets	(4.4)	(6.5)	(5.9)
Net cash provided by operating activities	703.9	566.0	507.2
Cash Flows From Investing Activities:
Increase in restricted cash and cash equivalents		0.0	0.0
Purchases of restricted securities available for sale	(43.8)	(34.5)	(39.1)
Proceeds from sale of restricted securities available for sale	19.7	27.8	35.9
Maturities of restricted securities available for sale	11.5	5.6	5.8
Principal collected on Loans receivable	2,576.7	2,189.5	1,955.8
Advances to Dealers	(2,414.8)	(1,968.3)	(1,881.3)
Purchases of Consumer Loans	(1,181.0)	(904.8)	(754.2)
Accelerated payments of Dealer Holdback	(52.6)	(47.1)	(53.6)
Payments of Dealer Holdback	(128.9)	(131.6)	(142.0)
Purchases of property and equipment	(25.1)	(8.4)	(5.5)
Net cash used in investing activities	(1,238.3)	(871.8)	(878.2)
Cash Flows From Financing Activities:
Borrowings under revolving secured line of credit	2,249.9	3,527.1	1,615.4
Repayments under revolving secured line of credit	(2,091.9)	(3,513.2)	(1,673.1)
Proceeds from secured financing	2,696.6	2,364.5	2,169.3
Repayments of secured financing	(2,116.9)	(1,907.5)	(1,575.8)
Proceeds from mortgage note	12.0	0.0	0.0
Payments of debt issuance costs and debt issuance costs	(13.7)	(14.6)	(9.0)
Repurchase of common stock	(129.1)	(123.5)	(121.7)
Tax benefits from stock-based compensation plans	0.0	0.0	27.2
Other financing activities	(7.0)	(2.5)	4.3
Net cash provided by financing activities	599.9	330.3	436.6
Net increase in cash and cash equivalents and restricted cash and cash equivalents	65.5	24.5	65.6
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	263.8	239.3	173.7
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	329.3	263.8	239.3
Supplemental Cash Flow Information [Abstract]
Cash paid during the period for interest	141.0	108.8	88.0
Cash paid during the period for income taxes	$ 168.8	$ 175.0	$ 111.2