Condensed Consolidated Financial Information Condensed Consolidated Cash Flow (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	$ 34,559	$ 34,680		$ 29,631	$ 24,246
Net cash provided by (used in) operating activities	148,782	69,217
Cash paid in acquisitions, net of cash received	(71,499)	(95)
Proceeds from disposition of franchises, property and equipment	2,940	2,207
Purchases of property and equipment, including real estate	(47,948)	(46,451)
Other	140	2,459
Net cash used in investing activities	(116,367)	(41,880)
Borrowings on credit facility - floorplan line and other	(1,656,192)	(1,594,371)	$ (1,656,192)
Repayments on credit facility - floorplan line and other	(1,702,447)	(1,598,291)	(1,702,447)
Borrowings on credit facility - acquisition line	(66,945)	(15,000)	(66,945)
Repayments on credit facility - acquisition line	(66,444)	(15,000)	(66,444)
Borrowings on other debt	65,614	0	$ 65,614
Principal payments on other debt	(40,078)	(232)
Borrowings Of Long Term Debt Related To Real Estate Loans, Net of Debt Issue Costs	46,835	0
Principal payments on debt related to real estate	(37,819)	(6,831)
Net Issuance of Common and Treasury Shares to Employee Stock Compensation Plans	(1,638)	(1,030)
Repurchases of common stock, amounts based on settlement date	(9,199)	0
Dividends paid	5,495	5,150
Borrowings (repayments) with subsidiaries	0	0
Investment in subsidiaries	0	0
Net cash used in financing activities	(27,534)	(17,163)
Effect of Exchange Rate on Cash and Cash Equivalents	47	260
Cash, cash equivalents, and restricted cash period increase (decrease)	4,928	10,434
CASH AND CASH EQUIVALENTS, end of period	33,090
Parent Company [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	0			0
Net cash provided by (used in) operating activities	35,814	33,939
Cash paid in acquisitions, net of cash received	0	0
Proceeds from disposition of franchises, property and equipment	0	0
Purchases of property and equipment, including real estate	0	0
Other	0	0
Net cash used in investing activities	0	0
Borrowings on credit facility - floorplan line and other	0	0
Repayments on credit facility - floorplan line and other	0	0
Borrowings on credit facility - acquisition line	(66,945)	(15,000)
Repayments on credit facility - acquisition line	(66,444)	(15,000)
Borrowings on other debt	0
Principal payments on other debt	(24,742)	0
Borrowings Of Long Term Debt Related To Real Estate Loans, Net of Debt Issue Costs	0
Principal payments on debt related to real estate	0	0
Net Issuance of Common and Treasury Shares to Employee Stock Compensation Plans	(1,638)	(1,030)
Repurchases of common stock, amounts based on settlement date	(9,199)
Dividends paid	5,495	(5,150)
Proceeds from (Payments for) Other Financing Activities	22,730
Borrowings (repayments) with subsidiaries		(70,250)
Investment in subsidiaries	(17,971)	(42,491)
Net cash used in financing activities	(35,814)	(33,939)
Effect of Exchange Rate on Cash and Cash Equivalents	0	0
Cash, cash equivalents, and restricted cash period increase (decrease)	0	0
CASH AND CASH EQUIVALENTS, end of period	0	0
Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	8,874	5,676		10,096	8,039
Net cash provided by (used in) operating activities	114,862	23,409
Cash paid in acquisitions, net of cash received	(31,414)	0
Proceeds from disposition of franchises, property and equipment	918	181
Purchases of property and equipment, including real estate	(34,101)	43,091
Other	140	(2,459)
Net cash used in investing activities	(64,457)	(40,451)
Borrowings on credit facility - floorplan line and other	(1,612,866)	(1,580,590)
Repayments on credit facility - floorplan line and other	(1,675,917)	(1,586,075)
Borrowings on credit facility - acquisition line	0	0
Repayments on credit facility - acquisition line	0	0
Borrowings on other debt	43,326
Principal payments on other debt	(1,987)	232
Borrowings Of Long Term Debt Related To Real Estate Loans, Net of Debt Issue Costs	34,817
Principal payments on debt related to real estate	(31,000)	(5,589)
Net Issuance of Common and Treasury Shares to Employee Stock Compensation Plans	0	0
Repurchases of common stock, amounts based on settlement date	0
Dividends paid	0	0
Proceeds from (Payments for) Other Financing Activities	(50,554)
Borrowings (repayments) with subsidiaries		68,483
Investment in subsidiaries	16,822	42,498
Net cash used in financing activities	(51,627)	14,679
Effect of Exchange Rate on Cash and Cash Equivalents	0	0
Cash, cash equivalents, and restricted cash period increase (decrease)	(1,222)	(2,363)
CASH AND CASH EQUIVALENTS, end of period	8,874
Non-Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	25,685	29,004		$ 19,535	$ 16,207
Net cash provided by (used in) operating activities	(1,894)	11,869
Cash paid in acquisitions, net of cash received	(40,085)	(95)
Proceeds from disposition of franchises, property and equipment	2,022	2,026
Purchases of property and equipment, including real estate	(13,847)	3,360
Other	0	0
Net cash used in investing activities	(51,910)	(1,429)
Borrowings on credit facility - floorplan line and other	(43,326)	(13,781)
Repayments on credit facility - floorplan line and other	(26,530)	(12,216)
Borrowings on credit facility - acquisition line	0	0
Repayments on credit facility - acquisition line	0	0
Borrowings on other debt	22,288
Principal payments on other debt	(13,349)	0
Borrowings Of Long Term Debt Related To Real Estate Loans, Net of Debt Issue Costs	12,018
Principal payments on debt related to real estate	(6,819)	(1,242)
Net Issuance of Common and Treasury Shares to Employee Stock Compensation Plans	0	0
Repurchases of common stock, amounts based on settlement date	0
Dividends paid	0	0
Proceeds from (Payments for) Other Financing Activities	27,824
Borrowings (repayments) with subsidiaries		1,767
Investment in subsidiaries	1,149	(7)
Net cash used in financing activities	59,907	2,097
Effect of Exchange Rate on Cash and Cash Equivalents	47	260
Cash, cash equivalents, and restricted cash period increase (decrease)	6,150	$ 12,797
CASH AND CASH EQUIVALENTS, end of period	$ 24,216