Consolidated Statements Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows From Operating Activities:
Net income	$ 656.1	$ 574.0	$ 470.2
Adjustments to reconcile cash provided by operating activities:
Provision for credit losses	76.4	56.9	129.3
Depreciation	7.3	5.4	6.0
Amortization	15.1	14.1	10.4
Increase (Decrease) in Deferred Income Taxes	85.5	49.3	(85.6)
Stock-based compensation	7.6	10.3	15.4
Loss on extinguishment of debt	1.8	0.0	0.0
Other Operating Activities, Cash Flow Statement	(0.2)	(0.2)	0.1
Change in operating assets and liabilities:
Increase in accounts payable and accrued liabilities	17.3	41.6	10.3
Increase in income taxes receivable	(58.3)	(5.7)	0.1
Increase (decrease) in income taxes payable	(2.3)	(37.4)	16.3
Increase in other assets	6.0	(4.4)	(6.5)
Net cash provided by operating activities	812.3	703.9	566.0
Cash Flows From Investing Activities:
Purchases of restricted securities available for sale	(40.1)	(43.8)	(34.5)
Proceeds from sale of restricted securities available for sale	29.1	19.7	27.8
Maturities of restricted securities available for sale	11.9	11.5	5.6
Principal collected on Loans receivable	2,971.2	2,576.7	2,189.5
Advances to Dealers	(2,424.5)	(2,414.8)	(1,968.3)
Purchases of Consumer Loans	(1,347.7)	(1,181.0)	(904.8)
Accelerated payments of Dealer Holdback	(58.8)	(52.6)	(47.1)
Payments of Dealer Holdback	(138.5)	(128.9)	(131.6)
Purchases of property and equipment	(26.8)	(25.1)	(8.4)
Net cash used in investing activities	(1,024.2)	(1,238.3)	(871.8)
Cash Flows From Financing Activities:
Borrowings under revolving secured line of credit	3,846.7	2,249.9	3,527.1
Repayments under revolving secured line of credit	(4,018.6)	(2,091.9)	(3,513.2)
Proceeds from secured financing	2,396.4	2,696.6	2,364.5
Repayments of secured financing	(2,149.5)	(2,116.9)	(1,907.5)
Proceeds from issuance of senior notes	800.0	0.0	0.0
Repayments of senior notes	(148.2)	0.0	0.0
Proceeds from mortgage note	0.0	12.0	0.0
Payments of debt issuance costs and debt issuance costs	(25.5)	(13.7)	(14.6)
Repurchase of common stock	(300.4)	(129.1)	(123.5)
Other financing activities	(0.6)	(7.0)	(2.5)
Net cash provided by financing activities	400.3	599.9	330.3
Net increase in cash and cash equivalents and restricted cash and cash equivalents	188.4	65.5	24.5
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	329.3	263.8	239.3
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	517.7	329.3	263.8
Supplemental Cash Flow Information [Abstract]
Cash paid during the period for interest	175.6	141.0	108.8
Cash paid during the period for income taxes	$ 172.4	$ 168.8	$ 175.0