Derivative Instruments and Risk Management Activities (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 581,700	$ 563,000
Notional amount of interest rate swaps entered into during current year	244,600
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(8,313)	(7,094)	$ (6,969)
Floorplan interest expense	$ 39,264	41,614	41,667
Interest Rate Swaps Latest Expiration	Oct. 30, 2022
Maximum Notional Amount Of Derivatives In Effect At Any Time	$ 779,700
Fair value of derivative financial instruments included in balance sheet as liabilities from interest risk management activities	31,153	28,653
Unrealized gain (loss) related to hedges, net of income taxes included in other comprehensive loss	(19,452)	$ (17,909)	(13,850)	$ (26,931)
Amount expected to be reclassified from other comprehensive loss into earnings	$ 11,800
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Average Fixed Interest Rate	2.70%	2.50%
Interest Rate Swaps Forward Starting [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 650,000
Derivative, Average Fixed Interest Rate	2.60%
Notional amount of interest rate swaps entered into during current year	$ 100,000
Number of additional forward interest rate swaps	13
Interest Rate Swaps expiring in 2016 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 151,600
Interest Rate Swaps expiring in 2017 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	251,600
Interest Rate Swaps expiring in 2018 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	151,600
Interest Rate Swaps expiring in 2019 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	1,600
Interest Rate Swaps expiring in 2020 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	1,600
Interest Rate Swaps expiring in 2021 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	9,900
Interest Rate Swaps expiring in 2022 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 13,700
Minimum [Member] | Interest Rate Swaps Forward Starting [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Inception Date	Dec. 30, 2016
Derivative, Maturity Date	Dec. 31, 2019
Maximum [Member] | Interest Rate Swaps Forward Starting [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Inception Date	Jan. 02, 2019
Derivative, Maturity Date	Dec. 31, 2021
Floorplan Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ 11,486	$ 9,837	$ 9,938