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Long-Term Debt - Additional Information (Detail) |  |  |  |  |  | 12 Months Ended
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Mar. 27, 2017 USD ($) | Mar. 10, 2017 USD ($) | Nov. 30, 2016 USD ($) | May 09, 2013 USD ($) | Jul. 02, 2012 USD ($) | Dec. 31, 2017 USD ($) Location $ / shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Loss on repurchase of debt instrument |  |  |  |  |  | $ 14,607,000 |  |
Minimum EBTDAR to rent ratio |  |  |  |  |  | 385.00% |  |
Incremental interest expense |  |  |  |  |  | $ 3,100,000 | $ 5,500,000 | $ 7,800,000
Net expense expected to be reclassified |  |  |  |  |  | 100,000 |  |
Derivative Instruments and Hedging Activities [Member] | Interest Rate Swap and Interest Rate Cap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative asset, fair value of interest rate swap and cap positions |  |  |  |  |  | 4,700,000 |  |
Derivative Instruments and Hedging Activities [Member] | Interest Rate Swap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative liability, fair value of interest rate swap and cap positions |  |  |  |  |  |  | 3,700,000 |
Derivative Instruments and Hedging Activities [Member] | Other Current Assets [Member] | Interest Rate Swap and Interest Rate Cap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative asset, fair value of interest rate swap and cap positions |  |  |  |  |  | 900,000 |  |
Derivative Instruments and Hedging Activities [Member] | Other Assets [Member] | Interest Rate Swap and Interest Rate Cap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative asset, fair value of interest rate swap and cap positions |  |  |  |  |  | 5,100,000 |  |
Derivative Instruments and Hedging Activities [Member] | Other Assets [Member] | Interest Rate Swap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative asset, fair value of interest rate swap and cap positions |  |  |  |  |  |  | 2,800,000 |
Derivative Instruments and Hedging Activities [Member] | Other Accrued Liabilities [Member] | Interest Rate Swap and Interest Rate Cap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative liability, fair value of interest rate swap and cap positions |  |  |  |  |  | 1,000,000 |  |
Derivative Instruments and Hedging Activities [Member] | Other Accrued Liabilities [Member] | Interest Rate Swap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative liability, fair value of interest rate swap and cap positions |  |  |  |  |  |  | 4,100,000 |
Derivative Instruments and Hedging Activities [Member] | Other Long-Term Liabilities [Member] | Interest Rate Swap and Interest Rate Cap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative liability, fair value of interest rate swap and cap positions |  |  |  |  |  | $ 300,000 |  |
Derivative Instruments and Hedging Activities [Member] | Other Long-Term Liabilities [Member] | Interest Rate Swap [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative liability, fair value of interest rate swap and cap positions |  |  |  |  |  |  | $ 2,400,000 |
Mortgage Notes [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt weighted average interest rate on note |  |  |  |  |  | 4.22% |  |
Number of operating locations related to mortgage financing | Location |  |  |  |  |  | 10 |  |
Mortgage financing aggregate |  |  |  |  |  | $ 52,500,000 |  |
Percentage of operating locations related to mortgage financing |  |  |  |  |  | 45.00% |  |
Outstanding mortgage principal balance |  |  |  |  |  | $ 419,700,000 |  |
Notes payable due date |  |  |  |  |  | between 2018 and 2033 |  |
Required Ratio [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Minimum EBTDAR to rent ratio |  |  |  |  |  | 150.00% |  |
2016 Credit Facility [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Maturity date of revolving credit facility and floor plan facility |  |  | Nov. 30, 2021 |  |  |  |  |
Minimum EBTDAR to rent ratio |  |  |  |  |  | 385.00% |  |
2016 Credit Facility [Member] | Required Ratio [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Minimum EBTDAR to rent ratio |  |  |  |  |  | 150.00% |  |
2016 Credit Facility [Member] | Minimum [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Net income and retained earnings free of restrictions |  |  |  |  |  | $ 148,700,000 |  |
2016 Revolving Credit Facility [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Increased availability under borrowing facility |  |  | $ 25,000,000 |  |  |  |  |
Current borrowing capacity |  |  |  |  |  | 250,000,000 |  |
Maximum borrowing capacity |  |  |  |  |  | 300,000,000 |  |
Borrowing base |  |  |  |  |  | 247,600,000 |  |
Letters of credit outstanding amount |  |  |  |  |  | 17,300,000 |  |
2016 Revolving Credit Facility |  |  |  |  |  | 75,000,000 |  |
Borrowing availability amount |  |  |  |  |  | 155,300,000 |  |
2016 Floor Plan Facilities [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Increased availability under borrowing facility |  |  | $ 215,000,000 |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  | 1,015,000,000 |  |
Increase in credit facility borrowing capacity |  |  |  |  |  | $ 1,265,000,000 |  |
2016 Used Vehicle Floor Plan Facility [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Allocation of credit facility increase, percentage |  |  |  |  |  | 30.00% |  |
7.0% Senior Subordinated Notes Due 2022 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Principal amount |  |  |  |  | $ 200,000,000 |  |  |
Stated interest rate on debt agreement |  |  |  |  | 7.00% | 7.00% | 7.00% |
Notes maturity date |  |  |  |  | Jul. 15, 2022 |  |  |
Cash paid to extinguish of debt instrument including early redemption premium and accrued and unpaid interest | $ 213,700,000 |  |  |  |  |  |  |
Loss on repurchase of debt instrument | $ 14,600,000 |  |  |  |  |  |  |
7.0% and 6.125% Senior Subordinated Notes [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Double-carry interest expense |  |  |  |  |  | $ 700,000 |  |
6.125% Senior Subordinated Notes due 2027 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Principal amount |  | $ 250,000,000 |  |  |  |  |  |
Stated interest rate on debt agreement |  | 6.125% |  |  |  | 6.125% | 6.125% |
Notes maturity date |  | Mar. 15, 2027 |  |  |  |  |  |
Notes issued at a price of principal amount |  | 100.00% |  |  |  |  |  |
Notes issued yield maturity, percentage |  | 6.125% |  |  |  |  |  |
Interest payable description |  |  |  |  |  | semi-annually in arrears on March 15 and September 15 of each year. |  |
Notes redemption price percentage of the principal amount |  |  |  |  |  | 100.00% |  |
Redemption Price |  |  |  |  |  | 106.125% |  |
Notes redemption price percentage of the par value due to change of control |  |  |  |  |  | 101.00% |  |
Debt instrument maximum allowed dividends per share | $ / shares |  |  |  |  |  | $ 0.12 |  |
Restrictive covenants under credit facilities and 6.125% notes |  |  |  |  |  | Specifically, the indenture governing the 6.125% Notes limits Sonic’s ability to pay quarterly cash dividends on its Class A and Class B common stock in excess of $0.12 per share. Sonic may only pay quarterly cash dividends on its Class A and Class B common stock if Sonic complies with the terms of the indenture governing the 6.125% Notes. |  |
Outstanding principal amount of the 6.125% notes |  |  |  |  |  | 25.00% |  |
6.125% Senior Subordinated Notes due 2027 [Member] | Maximum [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes redemption price percentage of the principal amount |  |  |  |  |  | 35.00% |  |
6.125% Senior Subordinated Notes due 2027 [Member] | Minimum [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Indebtedness with outstanding balance under other agreements |  |  |  |  |  | $ 50,000,000 |  |
5.0% Senior Subordinated Notes due 2023 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Principal amount |  |  |  | $ 300,000,000 |  |  |  |
Stated interest rate on debt agreement |  |  |  | 5.00% |  | 5.00% | 5.00% |
Notes maturity date |  |  |  | May 15, 2023 |  |  |  |
Notes issued at a price of principal amount |  |  |  | 100.00% |  |  |  |
Interest payable description |  |  |  |  |  | semi-annually in arrears on May 15 and November 15 of each year |  |
Notes redemption price percentage of the principal amount |  |  |  |  |  | 100.00% |  |
Notes redemption price percentage of the par value due to change of control |  |  |  |  |  | 101.00% |  |
Debt instrument maximum allowed dividends per share | $ / shares |  |  |  |  |  | $ 0.10 |  |
Restrictive covenants under credit facilities and 6.125% notes |  |  |  |  |  | Specifically, the indenture governing the 5.0% Notes limits Sonic’s ability to pay quarterly cash dividends on Sonic’s Class A and Class B common stock in excess of $0.10 per share. Sonic may only pay quarterly cash dividends on Sonic’s Class A and Class B common stock if Sonic complies with the terms of the indenture governing the 5.0% Notes. Sonic was in compliance with all restrictive covenants in the indenture governing the 5.0% Notes as of December 31, 2017. |  |
Outstanding principal amount of the 6.125% notes |  |  |  |  |  | 25.00% |  |
Outstanding notes repurchased amount |  |  |  |  |  |  | $ 10,700,000 |
Repurchase amount paid in cash, plus accrued and unpaid interest related thereto |  |  |  |  |  |  | $ 10,600,000 |
5.0% Senior Subordinated Notes due 2023 [Member] | Minimum [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Indebtedness with outstanding balance under other agreements |  |  |  |  |  | $ 50,000,000 |  |
9.0% Senior Subordinated Notes due 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Stated interest rate on debt agreement |  |  |  | 9.00% |  |  |  |
Convertible senior notes due |  |  |  | 2018 |  |  |  |

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