Consolidated Statements Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows From Operating Activities:
Net income	$ 332.8	$ 299.7	$ 266.2
Adjustments to reconcile cash provided by operating activities:
Provision for credit losses	90.2	41.5	12.8
Depreciation	6.1	5.7	5.5
Amortization	9.2	8.5	7.7
Loss on retirement of property and equipment	0.1	0.3	0.2
Provision for deferred income taxes	24.2	35.6	56.1
Loss on extinguishment of debt	0.0	0.0	21.8
Stock-based compensation	7.4	12.4	15.3
Change in operating assets and liabilities:
Increase in accounts payable and accrued liabilities	11.8	13.0	0.6
Increase in income taxes receivable	7.7	(8.6)	(0.3)
Increase (decrease) in income taxes payable	23.6	(2.9)	(17.0)
Increase in other assets	(5.9)	(1.0)	(3.7)
Net cash provided by operating activities	507.2	404.2	365.2
Cash Flows From Investing Activities:
Increase in restricted cash and cash equivalents	(57.3)	(9.8)	(46.3)
Purchases of restricted securities available for sale	(39.1)	(34.1)	(65.4)
Proceeds from sale of restricted securities available for sale	35.9	34.4	15.9
Maturities of restricted securities available for sale	5.8	4.1	49.6
Principal collected on Loans receivable	1,955.8	1,739.6	1,540.0
Advances to Dealers	(1,881.3)	(1,795.1)	(1,471.4)
Purchases of Consumer Loans	(754.2)	(371.9)	(204.3)
Accelerated payments of Dealer Holdback	(53.6)	(52.6)	(41.7)
Payments of Dealer Holdback	(142.0)	(150.1)	(135.5)
Purchases of property and equipment	(5.5)	(4.0)	(4.3)
Net cash used in investing activities	(935.5)	(639.5)	(363.4)
Cash Flows From Financing Activities:
Borrowings under revolving secured line of credit	1,615.4	1,657.0	2,796.2
Repayments under revolving secured line of credit	(1,673.1)	(1,718.8)	(2,779.5)
Proceeds from secured financing	2,169.3	1,247.0	1,754.7
Repayments of secured financing	(1,575.8)	(1,101.4)	(1,357.3)
Proceeds from issuance of senior notes	0.0	248.2	300.0
Repayments of senior notes	0.0	0.0	(350.0)
Payments of debt issuance costs and debt issuance costs	(9.0)	(11.0)	(30.4)
Repurchase of common stock	(121.7)	(86.5)	(343.7)
Proceeds from stock options exercised	0.0	0.0	0.6
Tax benefits from stock-based compensation plans	27.2	0.3	13.6
Other financing activities	4.3	0.4	(3.8)
Net cash provided by financing activities	436.6	235.2	0.4
Net increase (decrease) in cash and cash equivalents	8.3	(0.1)	2.2
Cash and cash equivalents, beginning of period	6.3	6.4	4.2
Cash and cash equivalents, end of period	14.6	6.3	6.4
Supplemental Cash Flow Information [Abstract]
Cash paid during the period for interest	88.0	61.8	55.2
Cash paid during the period for income taxes	$ 111.2	$ 146.9	$ 99.9