Interim Financial Statements - Assets Held For Sale, Discontinued Operations, Fair Value and Income Taxes (Details) $ in Millions	6 Months Ended	12 Months Ended
Jun. 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	$ 2,151.9	$ 2,163.2
Income Taxes
Corporate tax rate (as a percent)	21.00%	35.00%
Adjustment to the provisional estimate relating to the Tax Cuts and Jobs Act of 2017	$ 0.0
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	$ 546.4	$ 545.9
5.75% senior subordinated notes due 2022 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 559.6	$ 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	$ 297.4	$ 297.2
5.375% senior subordinated notes due 2024 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 292.8	$ 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	$ 494.7	$ 494.4
5.50% senior subordinated notes due 2026 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 484.9	$ 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	$ 297.2	$ 296.5
3.75% senior subordinated notes due 2020 \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	294.0	301.7
Mortgage facilities
Fair Value of Financial Instruments
Debt instrument, Carrying Value	245.8	235.5
Mortgage facilities \| Fair Value, Inputs, Level 2
Fair Value of Financial Instruments
Debt instrument, Fair Value	$ 243.2	$ 233.4
Retail Automotive Franchise
Divestitures
Number of franchises disposed \| item	6

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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