Derivative Instruments and Risk Management Activities - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) swap	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) swap	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Gain (loss) related to hedge ineffectiveness	$ 0	$ 0	$ 0	$ 0
Increase in floor plan interest expense due to impact of interest rate hedges	1,000,000	2,300,000	4,000,000	8,000,000
Floor plan interest expense	$ 14,685,000	13,491,000	43,335,000	38,659,000
Forward expiration date	Dec. 31, 2030
Maximum notional value of derivatives in effect at any time	$ 902,400,000		902,400,000
Accumulated unrealized gains (losses), net of income taxes	15,881,000	$ (5,800,000)	15,881,000	$ (5,800,000)	$ (674,000)	$ (9,331,000)
Amount expected to be reclassified from other comprehensive loss into earnings	1,500,000		1,500,000
Current liabilities from interest rate risk management activities	311,000		311,000		1,996,000
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	444,000		444,000		8,583,000
Derivative Liability	755,000		755,000		$ 10,579,000
Interest Rate Swaps
Derivative [Line Items]
Notional value	$ 803,900,000		$ 803,900,000
Weighted average interest rate	2.60%		2.60%
Number of additional forward interest rate swaps | swap	24		24
Forward Interest Rate Swaps
Derivative [Line Items]
Notional value	$ 375,000,000		$ 375,000,000
Weighted average interest rate	1.80%		1.80%
Number of additional forward interest rate swaps | swap	7		7
Forward Interest Rate Swaps | Minimum
Derivative [Line Items]
Forward start date	Dec. 31, 2018
Forward expiration date	Dec. 31, 2021
Forward Interest Rate Swaps | Maximum
Derivative [Line Items]
Forward start date	Dec. 31, 2020
Forward expiration date	Dec. 31, 2030