Derivative Instruments and Risk Management Activities (Details Textual) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) swap	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) swap	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Increase in floor plan interest expense due to impact of interest rate hedges	$ 2,800	$ 2,400	$ 8,300	$ 7,300
Floor Plan Interest Expense	9,685	10,452	$ 29,046	31,695
Forward swaps expiration date			Dec. 31, 2021
Fair value of derivative financial instruments included in balance sheet as liabilities from interest risk management activities	39,724		$ 39,724		$ 28,653
Amount expected to be reclassified from other comprehensive loss into earnings	13,300		13,300
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(24,828)	$ (14,344)	(24,828)	$ (14,344)	$ (17,909)	$ (13,850)
Interest Rate Swap [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Aggregate notional value of forward interest rate swaps	$ 562,300		$ 562,300
Derivative, Average Fixed Interest Rate	2.80%		2.80%
Interest Rate Swaps That Became Effective During Period [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Number of additional forward interest rate swaps	4		4
Aggregate notional value of forward interest rate swaps	$ 225,000		$ 225,000
Interest Rate Swaps That Expired During Period [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Number of additional forward interest rate swaps	8		8
Aggregate notional value of forward interest rate swaps	$ 225,000		$ 225,000
Interest Rate Swaps Forward Starting [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Number of additional forward interest rate swaps | swap	13		13
Aggregate notional value of forward interest rate swaps	$ 650,000		$ 650,000
Derivative, Average Fixed Interest Rate	2.60%		2.60%
Maximum Notional Amount Of Derivatives In Effect At Any Time	$ 761,000		$ 761,000
Minimum [Member] | Interest Rate Swaps Forward Starting [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Forward swaps start date			Dec. 31, 2016
Forward swaps expiration date			Dec. 31, 2019
Maximum [Member] | Interest Rate Swaps Forward Starting [Member]
Additional Derivative Instruments and Risk Management Activities (Textual) [Abstract]
Forward swaps start date			Jan. 02, 2019
Forward swaps expiration date			Dec. 31, 2021