Interim Financial Statements - Assets Held For Sale, Discontinued Operations and Fair Value (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Combined financial information regarding entities accounted for as discontinued operations
Number of entities newly classified as held for sale | item	0
Fair Value of Financial Instruments
Debt instrument, Carrying Value	$ 2,086.0	$ 2,163.2
5.75% senior subordinated notes due 2022
Fair Value of Financial Instruments
Interest rate (as a percent)	5.75%	5.75%
Debt instrument, Carrying Value	$ 546.6	$ 545.9
5.75% senior subordinated notes due 2022 | Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 557.5	$ 562.3
5.375% senior subordinated notes due 2024
Fair Value of Financial Instruments
Interest rate (as a percent)	5.375%	5.375%
Debt instrument, Carrying Value	$ 297.5	$ 297.2
5.375% senior subordinated notes due 2024 | Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 294.2	$ 300.2
5.50% senior subordinated notes due 2026
Fair Value of Financial Instruments
Interest rate (as a percent)	5.50%	5.50%
Debt instrument, Carrying Value	$ 494.9	$ 494.4
5.50% senior subordinated notes due 2026 | Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 491.4	$ 505.0
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	$ 297.5	$ 296.5
3.75% senior subordinated notes due 2020 | Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	296.8	301.7
Mortgage facilities
Fair Value of Financial Instruments
Debt instrument, Carrying Value	243.0	235.5
Mortgage facilities | Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 238.5	$ 233.4
Retail Automotive Franchise
Divestitures
Number of franchises disposed | item	6