Consolidated Statements Of Cash Flows (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash Flows From Operating Activities:
Net income	$ 266.2	$ 253.1	$ 219.7
Adjustments to reconcile cash provided by operating activities:
Provision for credit losses	12.8	21.9	24.0
Depreciation	5.5	5.4	5.1
Amortization	7.7	7.8	7.1
Loss on retirement of property and equipment	0.2	0.1	0
Provision for deferred income taxes	56.1	8.8	25.0
Gain (loss) on extinguishment of debt	21.8	0	0
Stock-based compensation	15.3	8.5	12.2
Change in operating assets and liabilities:
Increase in accounts payable and accrued liabilities	0.6	8.0	10.0
(Increase) decrease in income taxes receivable	(0.3)	0	(0.6)
Increase in income taxes payable	(17.0)	13.6	4.8
(Increase) decrease in other assets	(3.7)	(1.5)	1.3
Net cash provided by operating activities	365.2	325.7	308.6
Cash Flows From Investing Activities:
(Increase) decrease in restricted cash and cash equivalents	(46.3)	(18.9)	12.3
Purchases of restricted securities available for sale	(65.4)	(105.7)	(57.1)
Proceeds from sale of restricted securities available for sale	15.9	11.6	2.0
Maturities of restricted securities available for sale	49.6	86.2	9.6
Principal collected on Loans receivable	1,540.1	1,334.4	1,162.8
Advances to Dealers	(1,471.4)	(1,356.6)	(1,253.6)
Purchases of Consumer Loans	(204.3)	(124.0)	(108.8)
Accelerated payments of Dealer Holdback	(41.7)	(40.4)	(43.7)
Payments of Dealer Holdback	(135.5)	(114.2)	(115.7)
Net (increase) decrease in other loans	(0.1)	(0.4)	0.1
Purchases of property and equipment	(4.3)	(5.6)	(8.8)
Net cash used in investing activities	(363.4)	(333.6)	(400.9)
Cash Flows From Financing Activities:
Borrowings under revolving secured line of credit	2,796.2	2,816.6	2,507.4
Repayments under revolving secured line of credit	(2,779.5)	(2,757.3)	(2,507.8)
Proceeds from secured financing	1,754.7	1,004.7	1,742.0
Repayments of secured financing	(1,357.3)	(922.1)	(1,488.3)
Proceeds from issuance of senior notes	300.0	0	0
Repayments of senior notes	(350.0)	0	0
Principal payments under mortgage note	(3.8)	(0.2)	(0.3)
Payments of debt issuance costs	(30.4)	(5.3)	(6.5)
Repurchase of common stock	(343.7)	(135.2)	(152.5)
Proceeds from stock options exercised	0.6	0.6	0.6
Tax benefits from stock-based compensation plans	13.6	1.3	2.0
Net cash provided by financing activities	0.4	3.1	96.6
Net (decrease) increase in cash and cash equivalents	2.2	(4.8)	4.3
Cash and cash equivalents, beginning of period	4.2	9.0	4.7
Cash and cash equivalents, end of period	6.4	4.2	9.0
Supplemental Cash Flow Information [Abstract]
Cash paid during the period for interest	55.2	57.5	56.2
Cash paid during the period for income taxes	$ 99.9	$ 119.6	$ 92.4