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Interim Financial Statements - Assets Held For Sale, Discontinued Operations and Fair Value (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Combined financial information regarding entities accounted for as discontinued operations
Number of entities newly classified as held for sale \| item			0
Balance Sheet information regarding entities accounted for as discontinued operations
Total assets					$ 8.4
Total liabilities	$ 0.7		$ 0.7		6.1
Fair Value of Financial Instruments
Debt instrument, Carrying Value	$ 2,238.1		$ 2,238.1		$ 1,877.1
5.75% senior subordinated notes due 2022
Fair Value of Financial Instruments
Interest rate (as a percent)	5.75%		5.75%		5.75%
Debt instrument, Carrying Value	$ 545.7		$ 545.7		$ 545.1
5.75% senior subordinated notes due 2022 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 562.9		$ 562.9		$ 567.6
5.375% senior Subordinated Notes Due 2024
Fair Value of Financial Instruments
Interest rate (as a percent)	5.375%		5.375%		5.375%
Debt instrument, Carrying Value	$ 297.1		$ 297.1		$ 296.8
5.375% senior Subordinated Notes Due 2024 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 307.5		$ 307.5		$ 293.0
5.50% senior subordinated notes due 2026
Fair Value of Financial Instruments
Interest rate (as a percent)	5.50%		5.50%		5.50%
Debt instrument, Carrying Value	$ 494.2		$ 494.2		$ 493.7
5.50% senior subordinated notes due 2026 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 511.4		$ 511.4		489.4
3.75% senior subordinated notes due 2020
Fair Value of Financial Instruments
Interest rate (as a percent)	3.75%		3.75%
Debt instrument, Carrying Value	$ 296.2		$ 296.2
3.75% senior subordinated notes due 2020 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	302.1		302.1
Mortgage facilities
Fair Value of Financial Instruments
Debt instrument, Carrying Value	211.8		211.8		199.9
Mortgage facilities \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	210.2		$ 210.2		195.6
Retail Automotive Franchise
Divestitures
Number of franchises disposed \| item			12
Retail Automotive Franchise \| Disposed of by sale - Discontinued Operations
Divestitures
Number of franchises disposed \| item			1
Retail Automotive Franchise \| Disposal group, not discontinued operations
Divestitures
Number of franchises disposed \| item			11
Retail Automotive Franchise \| Held-for-sale
Combined financial information regarding entities accounted for as discontinued operations
Revenues		$ 5.6	$ 2.1	$ 19.6
Pre-tax gain (loss)	0.1	$ 0.3	(0.5)	$ (1.7)
Balance Sheet information regarding entities accounted for as discontinued operations
Inventories					4.6
Other assets					3.8
Total assets					8.4
Floor plan notes payable (including non-trade)					4.8
Other liabilities	0.7		0.7		1.3
Total liabilities	$ 0.7		$ 0.7		$ 6.1