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Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) |  | 12 Months Ended
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Jan. 01, 2018 USD ($) | Dec. 31, 2017 USD ($) Dealerships State Brand Collision Store Segment Franchise | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Number of new vehicle dealerships | Dealerships |  | 114 |  |
Number of states | State |  | 13 |  |
Number of different brands of cars and light trucks | Brand |  | 23 |  |
Number of collision repair centers | Collision |  | 18 |  |
Number of pre-owned stores | Store |  | 9 |  |
Number of operating segments | Segment |  | 2 |  |
Book overdraft position |  | $ 6,900,000 | $ 8,000,000 |
Revenue allowances for commission reserves |  | 20,900,000 | 19,200,000 |
Amount recognized for floor plan assistance |  | $ 45,300,000 | 45,000,000 | $ 42,100,000
Term for funding of finance contracts |  | 10 days |  |
Contracts in transit included in receivables, net |  | $ 267,600,000 | 236,400,000 |
Number of reporting units | Franchise |  | 2 |  |
Goodwill |  | $ 525,780,000 | 472,437,000 | 471,493,000
Insurance reserves |  | 22,000,000 | 22,700,000 |
Concentrations of credit risk with respect to receivables are limited primarily to receivables from automobile manufacturers |  | 102,500,000 | 92,800,000 |
Deposits |  | 3,000,000 | 10,000,000 |
Reduction in interest expense |  | 500,000 | 600,000 | 1,500,000
Advertising expense |  | 61,600,000 | 61,700,000 | 61,600,000
Cooperative manufacturer credits advertising expenses |  | $ 26,000,000 | 26,200,000 | 24,200,000
Number of reportable Segment | Segment |  | 2 |  |
Financial Institutions [Member] |  |  |  |
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Concentrations of credit risk with respect to receivables are limited primarily to receivables from financial institutions |  | $ 299,600,000 | 265,300,000 |
Franchise Assets [Member] |  |  |  |
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Franchise asset impairment charge |  | 3,600,000 |  |
Continuing Operations [Member] | Franchise Assets [Member] |  |  |  |
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Franchise asset impairment charge |  | $ 3,600,000 | $ 0 | $ 3,300,000
Number of franchises resulted in asset impairment charge | Franchise |  | 4 |  |
Accounting Standards Update (“ASU”) 2014-09 [Member] | Subsequent Event [Member] |  |  |  |
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Cumulative effect adjustment to increase retained earnings, net | $ 3,000,000 |  |  |
Dealership [Member] |  |  |  |
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Percentage of dealership that is accounted for under the equity method |  | 50.00% |  |
Franchised Dealerships Segment [Member] |  |  |  |
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Number of stores | Store |  | 103 |  |
Goodwill |  | $ 465,800,000 |  |
Pre-Owned Stores Segment [Member] |  |  |  |
Description Of Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Number of stores | Store |  | 9 |  |
Goodwill |  | $ 60,000,000 |  |

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