Derivative Instruments and Risk Management Activities (Details Textual)	3 Months Ended
	Mar. 31, 2018 USD ($) swap	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Derivative Asset, Noncurrent	$ 15,987,000		$ 9,501,000
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	0	$ 0
Increase in floor plan interest expense due to impact of interest rate hedges	1,700,000	2,900,000
Floor Plan Interest Expense	$ 14,087,000	11,942,000
Forward swaps expiration date	Dec. 31, 2030
Maximum Notional Amount Of Derivatives In Effect At Any Time	$ 918,400,000
Derivative Asset	15,987,000		9,501,000
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	8,731,000	$ (6,673,000)	(674,000)	$ (9,331,000)
Amount expected to be reclassified from other comprehensive loss into earnings	2,700,000
Derivative Liability, Current	1,177,000		1,996,000
Derivative Liability, Noncurrent	3,512,000		8,583,000
Derivative Liability	4,689,000		10,579,000
Interest Rate Swap [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Aggregate notional value of forward interest rate swaps	$ 822,100,000
Derivative, Average Fixed Interest Rate	2.60%
Number of additional forward interest rate swaps | swap	25
Interest Rate Swaps Forward Starting [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Aggregate notional value of forward interest rate swaps	$ 375,000,000
Derivative, Average Fixed Interest Rate	0.00%
Number of additional forward interest rate swaps | swap	7
Minimum [Member] | Interest Rate Swaps Forward Starting [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Forward swaps start date	Dec. 31, 2018
Forward swaps expiration date	Dec. 31, 2021
Maximum [Member] | Interest Rate Swaps Forward Starting [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Forward swaps start date	Dec. 31, 2020
Forward swaps expiration date	Dec. 31, 2030
Level 2 [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Interest Rate Derivative Assets, at Fair Value			$ 9,501,000