Summary of Significant Accounting Policies (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) store	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 01, 2017
Debt Instrument [Line Items]
Number of stores | store	171
Number of new vehicle brands	30
Number of states	18
Interest costs capitalized	$ (500)	$ (400)	$ (500)
Cumulative effect of new accounting principle on additional paid in capital	0
Reclassification between financing and operating cash flows	148,856	90,905	79,551
Reclassification between financing and operating cash flows	(396,313)	(266,062)	(105,292)
Long-term debt, outstanding amount	2,300,000
Accrued warranty balance	$ 400	400
Number of reportable segments	3
Minimum
Debt Instrument [Line Items]
Period that contracts in transit are outstanding	5 days
Long-term debt, variable interest rate (in percent)	2.75%
Maximum
Debt Instrument [Line Items]
Period that contracts in transit are outstanding	10 days
Long-term debt, variable interest rate (in percent)	4.25%
Accounting Standards Update 2016-09
Debt Instrument [Line Items]
Reclassification between financing and operating cash flows		4,400	5,000
Reclassification between financing and operating cash flows		4,400	$ 5,000
Tax-affected state net operating loss carryforwards		300
Credit Facility | Line of Credit
Debt Instrument [Line Items]
Number of financial institutions				18
Number of manufacturer affiliated finance companies				7
Retained Earnings
Debt Instrument [Line Items]
Cumulative effect of new accounting principle on additional paid in capital	$ 169
Retained Earnings | Accounting Standards Update 2016-09
Debt Instrument [Line Items]
Cumulative effect of new accounting principle on additional paid in capital		200
Additional Paid-In Capital
Debt Instrument [Line Items]
Cumulative effect of new accounting principle on additional paid in capital	$ (169)
Additional Paid-In Capital | Accounting Standards Update 2016-09
Debt Instrument [Line Items]
Cumulative effect of new accounting principle on additional paid in capital		$ (200)