Acquisitions - Assets (Liabilities) Acquired (Assumed) at Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Assets (liabilities) acquired (assumed) at fair value:
Goodwill	$ 386,915		$ 359,117
Cash paid for acquisition, net of cash acquired	$ 48,408	$ 82,195
Assets Or Stock Of Multiple Dealership Locations Acquired | Membership and customer lists
Assets (liabilities) acquired (assumed) at fair value:
Intangible assets acquired		$ 766
Assets Or Stock Of Multiple Dealership Locations Acquired | Membership and customer lists | Minimum
Assets (liabilities) acquired (assumed) at fair value:
Estimated Life (in years)	4 years	4 years
Assets Or Stock Of Multiple Dealership Locations Acquired | Membership and customer lists | Maximum
Assets (liabilities) acquired (assumed) at fair value:
Estimated Life (in years)	7 years	7 years
2019 Acquisitions
Assets (liabilities) acquired (assumed) at fair value:
Inventory, net	$ 19,871
Prepaid expenses and other assets	95
Property and equipment, net	360
Accounts payable	(2)
Accounts payable and accrued liabilities	(113)
Total tangible net assets acquired	20,211
Goodwill	28,198
Purchase Price	48,409
Cash paid for acquisition, net of cash acquired	48,409
Inventory purchases financed via floor plan	(13,854)
Cash payment net of floor plan financing	$ 34,555
2018 Acquisitions
Assets (liabilities) acquired (assumed) at fair value:
Cash and cash equivalents		$ 2,648
Contracts in Transit		103
Accounts receivable, net		103
Inventory, net		40,856
Prepaid expenses and other assets		155
Property and equipment, net		818
Other assets		48
Accounts payable		(64)
Accounts payable and accrued liabilities		(1,440)
Other liabilities		(168)
Total tangible net assets acquired		43,059
Intangible assets acquired		1,084
Goodwill		40,725
Purchase Price		84,868
Cash and cash equivalents acquired		(2,648)
Cash paid for acquisition, net of cash acquired		82,220
Inventory purchases financed via floor plan		(29,365)
Cash payment net of floor plan financing		52,855
2018 Acquisitions | Trademarks and trade names
Assets (liabilities) acquired (assumed) at fair value:
Intangible assets acquired		$ 318
Estimated Life (in years)		15 years