Long-Term Debt Revolving Loan (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2014	Jul. 31, 2013
Debt Instrument [Line Items]
Debt, long-term and short-term	$ 3,192,400,000			$ 3,192,400,000
Gains (Losses) on Extinguishment of Debt	90,700,000	64,000,000	17,200,000
Deferred Finance Costs, Noncurrent, Net	33,000,000	30,100,000	53,200,000	33,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt, long-term and short-term	0	0		0
Letters of Credit Outstanding, Amount	2,100,000			2,100,000
Floorplan sub-facility, reserve	312,300,000			312,300,000
Line of Credit Facility, Maximum Borrowing Capacity	1,250,000,000.0			1,250,000,000.0
Line of Credit Facility, Fee on Unused Portion, Basis Points	25			25
Line of Credit Facility, Decrease in Applicable Interest Rate Margin, Basis Points					50
Gains (Losses) on Extinguishment of Debt	400,000
Deferred Finance Costs, Noncurrent, Net	6,400,000			6,400,000
Variable Interest Rate Margin, ABR Determination, Basis Point Plus Federal Funds Effective Rate	50			50
Variable Interest Rate Margin, ABR Determination, Percentage Plus LIBOR	1.00%			1.00%
Potential Margin Reduction	0.25%			0.25%
Line of Credit Facility, Borrowing Base	1,253,400,000			1,253,400,000
Line of Credit Facility, Remaining Borrowing Capacity	935,600,000			935,600,000
Revolving Loan, Consecutive Business Days Below Minimum Average Daily Excess Cash Availability Requirement	5 days
Revolving Loan, Minimum Average Daily Excess Cash Availability Requirement, Consecutive Business Days Met	30 days
Line Of Credit Facility, Maximum Aggregate Increase	300,000,000			300,000,000
Revolving Loan Maturity Acceleration Provision, Days Prior to Maturity				45 days
Revolving Loan Maturity Acceleration Provision, Excess Cash Availability Limit, Amount Plus Maturing Debt	150,000,000			150,000,000
Minimum Liquidity Condition, Cash Availability	125,000,000			125,000,000
Cash Availability, Percentage of Borrowing Base	10.00%			10.00%
Minimum Liquidity Condition, Amount	100,000,000			100,000,000
Fixed Charge Coverage Ratio	1.00			1.00
Previous Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	900,000,000			900,000,000
Line Of Credit Facility, Maximum Aggregate Increase	200,000,000			200,000,000
Term Loan [Member]
Debt Instrument [Line Items]
Debt, long-term and short-term	1,513,500,000	1,528,900,000		1,513,500,000
Gains (Losses) on Extinguishment of Debt		10,300,000
Deferred Finance Costs, Net						6,100,000
Higher Utilization [Member] | Previous Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Fee on Unused Portion, Basis Points	37.5			37.5
Lower Utilization [Member] | Previous Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Fee on Unused Portion, Basis Points	50			50
LIBOR [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	1.50%			1.50%
LIBOR [Member] | Minimum [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	2.25%			2.25%
LIBOR [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	2.00%			2.00%
LIBOR [Member] | Maximum [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	2.50%			2.50%
ABR [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	0.50%			0.50%
ABR [Member] | Minimum [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	1.25%			1.25%
ABR [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	1.00%			1.00%
ABR [Member] | Maximum [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin	1.50%			1.50%
Accounts Payable, Inventory Financing [Member]
Debt Instrument [Line Items]
Revolving Loan financing agreement	330,100,000	256,100,000		330,100,000
Accounts Payable, Inventory Financing [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Amount owed under Revolving loan financing agreement	312,300,000			312,300,000
Floorplan sub-facility, variation in balance due to timing	17,800,000			17,800,000
Accounts Payable, Inventory Financing [Member] | Previous Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Floorplan sub-facility	$ 400,000,000			$ 400,000,000
Percentage reserve for open orders under financing agreement					15.00%