Derivative Instruments and Risk Management Activities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivatives in cash flow hedging relationship
Interest rate swap contracts	$ (5,621)	$ 1,804	$ (16,763)	$ (4,709)
Location of Loss Reclassified from OCI into Statements of Operations
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ (2,141)	$ (1,980)	(4,256)	(3,947)
Floorplan Interest Expense [Member]
Location of Loss Reclassified from OCI into Statements of Operations
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax			5,623	5,470
Other Interest Expense [Member]
Location of Loss Reclassified from OCI into Statements of Operations
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax			$ 1,186	845
Interest Rate Swap Contracts [Member]
Derivatives in cash flow hedging relationship
Interest rate swap contracts				$ (4,709)