Derivative Instruments and Risk Management Activities (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 02, 2018	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 623,000,000	$ 765,300,000
Notional amount of interest rate swaps entered into during current year	110,600,000
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(7,623,000)	(8,393,000)	$ (8,313,000)
Floorplan interest expense	$ 52,372,000	44,927,000	39,264,000
Derivative, Maturity Date	Dec. 31, 2030
Maximum Notional Amount Of Derivatives In Effect At Any Time	$ 918,400,000
Unrealized gain (loss) related to hedges, net of income taxes included in other comprehensive loss	(674,000)	(9,331,000)	(19,452,000)		$ (17,909,000)
Amount expected to be reclassified from other comprehensive loss into earnings	6,900,000
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	$ 0
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Average Fixed Interest Rate	2.50%
Interest Rate Swaps Forward Starting [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 575,000,000
Derivative, Average Fixed Interest Rate	2.10%
Number of additional forward interest rate swaps	11
Interest Rate Swaps expiring in 2017 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 153,600,000
Interest Rate Swaps expiring in 2018 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	353,700,000
Interest Rate Swaps expiring in 2019 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	53,700,000
Interest Rate Swaps expiring in 2020 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	12,000,000
Interest Rate Swaps expiring in 2021 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	18,800,000
Interest Rate Swaps expiring in 2022 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	26,900,000
Expires in 2023 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 4,200,000
Minimum [Member] | Interest Rate Swaps Forward Starting [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Inception Date	Jan. 01, 2018
Derivative, Maturity Date	Dec. 31, 2020
Maximum [Member] | Interest Rate Swaps Forward Starting [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Inception Date	Dec. 31, 2020
Derivative, Maturity Date	Dec. 31, 2030
Floorplan Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ 10,272,000	11,097,000	$ 11,486,000
Fair Value, Inputs, Level 2 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value of derivative financial instruments included in balance sheet as liabilities from interest risk management activities	10,579,000	24,411,000
Interest Rate Derivative Assets, at Fair Value	$ 9,501,000	$ 9,484,000
Scenario, Forecast [Member] | Interest Rate Swaps Forward Starting [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount				$ 200,000,000