Consolidated Statements Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities:
Net income	$ 470.2	$ 332.8	$ 299.7
Adjustments to reconcile cash provided by operating activities:
Provision for credit losses	129.3	90.2	41.5
Depreciation	6.0	6.1	5.7
Amortization	10.4	9.2	8.5
Loss on retirement of property and equipment	0.1	0.1	0.3
Increase (Decrease) in Deferred Income Taxes	(85.6)	24.2	35.6
Stock-based compensation	15.4	7.4	12.4
Change in operating assets and liabilities:
Increase in accounts payable and accrued liabilities	10.3	11.8	13.0
Increase in income taxes receivable	0.1	7.7	(8.6)
Increase (decrease) in income taxes payable	16.3	23.6	(2.9)
Increase in other assets	(6.5)	(5.9)	(1.0)
Net cash provided by operating activities	566.0	507.2	404.2
Cash Flows From Investing Activities:
Increase in restricted cash and cash equivalents	(30.9)	(57.3)	(9.8)
Purchases of restricted securities available for sale	(34.5)	(39.1)	(34.1)
Proceeds from sale of restricted securities available for sale	27.8	35.9	34.4
Maturities of restricted securities available for sale	5.6	5.8	4.1
Principal collected on Loans receivable	2,189.5	1,955.8	1,739.6
Advances to Dealers	(1,968.3)	(1,881.3)	(1,795.1)
Purchases of Consumer Loans	(904.8)	(754.2)	(371.9)
Accelerated payments of Dealer Holdback	(47.1)	(53.6)	(52.6)
Payments of Dealer Holdback	(131.6)	(142.0)	(150.1)
Purchases of property and equipment	(8.4)	(5.5)	(4.0)
Net cash used in investing activities	(902.7)	(935.5)	(639.5)
Cash Flows From Financing Activities:
Borrowings under revolving secured line of credit	3,527.1	1,615.4	1,657.0
Repayments under revolving secured line of credit	(3,513.2)	(1,673.1)	(1,718.8)
Proceeds from secured financing	2,364.5	2,169.3	1,247.0
Repayments of secured financing	(1,907.5)	(1,575.8)	(1,101.4)
Proceeds from issuance of senior notes	0.0	0.0	248.2
Payments of debt issuance costs and debt issuance costs	(14.6)	(9.0)	(11.0)
Repurchase of common stock	(123.5)	(121.7)	(86.5)
Tax benefits from stock-based compensation plans	0.0	27.2	0.3
Other financing activities	(2.5)	4.3	0.4
Net cash provided by financing activities	330.3	436.6	235.2
Net increase (decrease) in cash and cash equivalents	(6.4)	8.3	(0.1)
Cash and cash equivalents, beginning of period	14.6	6.3	6.4
Cash and cash equivalents, end of period	8.2	14.6	6.3
Supplemental Cash Flow Information [Abstract]
Cash paid during the period for interest	108.8	88.0	61.8
Cash paid during the period for income taxes	$ 175.0	$ 111.2	$ 146.9