Derivative Instruments and Risk Management Activities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Asset	$ 15,987		$ 9,501
Derivative Asset, Noncurrent	15,987		9,501
Derivative Liability, Noncurrent	3,512		8,583
Derivative Liability, Current	1,177		1,996
Derivatives in cash flow hedging relationship
Interest rate swap contracts	7,892	$ 390
Location of Loss Reclassified from OCI into Statements of Operations
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(1,513)	(2,268)
Derivative Liability	4,689		$ 10,579
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	1,737	2,948
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	$ (254)	681
Interest Rate Swap Contracts [Member]
Derivatives in cash flow hedging relationship
Interest rate swap contracts		$ 390