Derivative Instruments and Risk Management Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives in cash flow hedging relationship
Interest rate swap contracts	$ 1,034	$ 1,728	$ (9,856)
Location of Loss Reclassified from OCI into Statements of Operations
Income (Loss) Reclassified from OCI into Statements of Operations	7,623	8,393	8,313
Floorplan Interest Expense [Member]
Location of Loss Reclassified from OCI into Statements of Operations
Income (Loss) Reclassified from OCI into Statements of Operations	(10,272)	(11,097)	(11,486)
Other Interest Expense [Member]
Location of Loss Reclassified from OCI into Statements of Operations
Income (Loss) Reclassified from OCI into Statements of Operations	$ (1,924)	$ (2,332)	$ (1,814)