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Derivative Instruments and Risk Management Activities - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) swap	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) swap	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Gain (loss) related to hedge ineffectiveness	$ 0	$ 0	$ 0	$ 0
Increase in floor plan interest expense due to impact of interest rate hedges	1,300,000	2,700,000	3,000,000	5,700,000
Floor plan interest expense	$ 14,563,000	13,226,000	28,650,000	25,168,000
Forward expiration date	Dec. 31, 2030
Maximum notional value of derivatives in effect at any time	$ 902,400,000		902,400,000
Accumulated unrealized gains (losses), net of income taxes	12,509,000	$ (7,255,000)	12,509,000	$ (7,255,000)	$ (674,000)	$ (9,331,000)
Amount expected to be reclassified from other comprehensive loss into earnings	200,000		200,000
Interest Rate Swaps
Derivative [Line Items]
Notional value	$ 804,600,000		$ 804,600,000
Weighted average interest rate	2.60%		2.60%
Number of additional forward interest rate swaps | swap	24		24
Forward Interest Rate Swaps
Derivative [Line Items]
Notional value	$ 375,000,000		$ 375,000,000
Weighted average interest rate	1.80%		1.80%
Number of additional forward interest rate swaps | swap	7		7
Forward Interest Rate Swaps | Minimum
Derivative [Line Items]
Forward start date	Dec. 31, 2018
Forward expiration date	Dec. 31, 2021
Forward Interest Rate Swaps | Maximum
Derivative [Line Items]
Forward start date	Dec. 31, 2020
Forward expiration date	Dec. 31, 2030

X

- Definition

Interest expense incurred on borrowings under the floorplan line of the credit facility and arrangements with vehicle manufacturer affiliates for the financing of specific new, used and rental vehicle inventory. Interest expense also includes the net impact of interest rate swap settlements during the period, which are designed to hedge exposure to variable interest rates.

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- Definition

The increase or decrease in floorplan interest expense due to interest rate hedges.

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- Definition

Maximum notional amount of derivatives in effect at any time.

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580

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- Definition

Average fixed interest rate related to the group of interest rate derivatives.

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- Definition

Date the entity entered into the derivative contract, in CCYY-MM-DD format.

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Date the derivative contract matures, in CCYY-MM-DD format.

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The number of derivative instruments of a particular group held by the entity.

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The net gain (loss) recognized in earnings during the reporting period representing the amount of the cash flow hedges' ineffectiveness.

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The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

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