Acquisitions - Summary of Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Cash paid, net of cash acquired	$ 366.6	$ 373.8	$ 460.4
Debt issued		125.1
Total	393.0	498.9
Goodwill	454.6	434.9	$ 256.3
Acquisitions 2019
Business Acquisition [Line Items]
Trade receivables, net	0.0
Inventories	105.2
Franchise value	0.0
Property and equipment	124.0
Other assets	193.1
Floor plan notes payable	0.0
Other liabilities	(29.3)
Assets acquired and liabilities assumed, excluding goodwill	393.0
Goodwill	0.0
Assets acquired and liabilities assumed, net	$ 393.0
Acquisitions 2018
Business Acquisition [Line Items]
Trade receivables, net		0.7
Inventories		180.0
Franchise value		29.8
Property and equipment		179.7
Other assets		48.6
Floor plan notes payable		(10.8)
Other liabilities		(2.3)
Assets acquired and liabilities assumed, excluding goodwill		425.7
Goodwill		73.2
Assets acquired and liabilities assumed, net		$ 498.9