Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Business	Business Genuine Parts Company and all of its majority-owned subsidiaries (the Company) is a distributor of automotive replacement parts, industrial parts and materials and business products. The Company serves a diverse customer base through approximately 3,100 locations in North America, Australasia and Europe and, therefore, has limited exposure from credit losses to any particular customer, region, or industry segment. The Company performs periodic credit evaluations of its customers’ financial condition and generally does not require collateral. The Company has evaluated subsequent events through the date the financial statements were issued.
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include all of the accounts of the Company. The net income attributable to noncontrolling interests is not material to the Company’s consolidated net income. Intercompany accounts and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of the consolidated financial statements, in conformity with U.S. generally accepted accounting principles, requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results may differ from those estimates and the differences could be material.
Revenue Recognition	Revenue Recognition The Company records revenue when the following criteria are met: persuasive evidence of an arrangement exists, delivery has occurred, the Company’s price to the customer is fixed and determinable and collectability is reasonably assured. Delivery is not considered to have occurred until the customer assumes the risks and rewards of ownership.
Foreign Currency Translation	Foreign Currency Translation The consolidated balance sheets and statements of income and comprehensive income of the Company’s foreign subsidiaries have been translated into U.S. dollars at the current and average exchange rates, respectively. The foreign currency translation adjustment is included as a component of accumulated other comprehensive loss.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less when purchased to be cash equivalents.
Trade Accounts Receivable and the Allowance for Doubtful Accounts	Trade Accounts Receivable and the Allowance for Doubtful Accounts The Company evaluates the collectability of trade accounts receivable based on a combination of factors. The Company estimates an allowance for doubtful accounts as a percentage of net sales based on historical bad debt experience and periodically adjusts this estimate when the Company becomes aware of a specific customer’s inability to meet its financial obligations (e.g., bankruptcy filing) or as a result of changes in the overall aging of accounts receivable. While the Company has a large customer base that is geographically dispersed, a general economic downturn in any of the industry segments in which the Company operates could result in higher than expected defaults and, therefore, the need to revise estimates for bad debts.
Merchandise Inventories, Including Consideration Received From Vendors	Merchandise Inventories, Including Consideration Received From Vendors Merchandise inventories are valued at the lower of cost or market. Cost is determined by the last-in, first-out (LIFO) method for a majority of U.S. automotive parts, electrical/electronic materials, and industrial parts, and by the first-in, first-out (FIFO) method for business products and certain non-U.S. and other inventories. If the FIFO method had been used for all inventories, cost would have been approximately $440,550,000 and $426,760,000 higher than reported at December 31, 2017 and 2016, respectively. During 2017 and 2016, reductions in industrial parts inventories resulted in liquidations of LIFO inventory layers. The effects of the LIFO liquidations in 2017 and 2016 reduced cost of goods sold by approximately $2,000,000 and $6,000,000, respectively. There were no LIFO liquidations in 2015. The Company identifies slow moving or obsolete inventories and estimates appropriate provisions related thereto. Historically, these losses have not been significant as the vast majority of the Company’s inventories are not highly susceptible to obsolescence and are eligible for return under various vendor return programs. While the Company has no reason to believe its inventory return privileges will be discontinued in the future, its risk of loss associated with obsolete or slow moving inventories would increase if such were to occur. The Company enters into agreements at the beginning of each year with many of its vendors that provide for inventory purchase incentives. Generally, the Company earns inventory purchase incentives upon achieving specified volume purchasing levels or other criteria. The Company accrues for the receipt of these incentives as part of its inventory cost based on cumulative purchases of inventory to date and projected inventory purchases through the end of the year. While management believes the Company will continue to receive consideration from vendors in 2018 and beyond, there can be no assurance that vendors will continue to provide comparable amounts of incentives in the future.
Prepaid Expenses and Other Current Assets	Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist primarily of prepaid expenses, amounts due from vendors, and income taxes receivable.
Goodwill	Goodwill The Company reviews its goodwill annually in the fourth quarter, or sooner if circumstances indicate that the carrying amount may exceed fair value. The Company tests goodwill for impairment at the reporting unit level, which is an operating segment or a level below an operating segment, which is referred to as a component. A component of an operating segment is a reporting unit if the component constitutes a business for which discrete financial information is available and management regularly reviews the operating results of that component. However, two or more components of an operating segment are aggregated and deemed a single reporting unit if the components have similar economic characteristics. A combination of qualitative assessments and present value of future cash flows approaches was used to determine any potential impairment.
Property, Plant, and Equipment	Property, Plant, and Equipment Property, plant, and equipment are stated at cost. Depreciation and amortization are primarily determined on a straight-line basis over the following estimated useful lives of each asset: buildings and improvements, 10 to 40 years; machinery and equipment, 5 to 15 years.
Long-Lived Assets Other Than Goodwill	Long-Lived Assets Other Than Goodwill The Company assesses its long-lived assets other than goodwill for impairment whenever facts and circumstances indicate that the carrying amount may not be fully recoverable. To analyze recoverability, the Company projects undiscounted net future cash flows over the remaining life of such assets. If these projected cash flows are less than the carrying amount, an impairment would be recognized, resulting in a write-down of assets with a corresponding charge to earnings. Impairment losses, if any, are measured based upon the difference between the carrying amount and the fair value of the assets.
Self-Insurance	Self-Insurance The Company is self-insured for the majority of group health insurance costs. A reserve for claims incurred but not reported is developed by analyzing historical claims data provided by the Company’s claims administrators. These reserves are included in accrued expenses in the accompanying consolidated balance sheets as the expenses are expected to be paid within one year. Long-term insurance liabilities consist primarily of reserves for the workers’ compensation program. In addition, the Company carries various large risk deductible workers’ compensation policies for the majority of workers’ compensation liabilities. The Company records the workers’ compensation reserves based on an analysis performed by an independent actuary. The analysis calculates development factors, which are applied to total reserves as provided by the various insurance companies who underwrite the program. While the Company believes that the assumptions used to calculate these liabilities are appropriate, significant differences in actual experience or significant changes in these assumptions may materially affect workers’ compensation costs.
Business Combinations	Business Combinations From time to time, the Company enters into business combinations. The Company recognizes the identifiable assets acquired, the liabilities assumed, and any noncontrolling interests in an acquiree at their fair values as of the date of acquisition. The Company measures goodwill as the excess of consideration transferred, which the Company also measures at fair value, over the net of the acquisition date fair values of the identifiable assets acquired and liabilities assumed. The acquisition method of accounting requires the Company to make significant estimates and assumptions regarding the fair values of the elements of a business combination as of the date of acquisition, including the fair values of identifiable intangible assets, deferred tax asset valuation allowances, liabilities including those related to debt, pensions and other postretirement plans, uncertain tax positions, contingent consideration and contingencies. This method also requires the Company to refine these estimates over a measurement period not to exceed one year to reflect new information obtained about facts and circumstances that existed as of the acquisition date that, if known, would have affected the measurement of the amounts recognized as of that date. If the Company is required to adjust provisional amounts that were recorded for the fair values of assets and liabilities in connection with acquisitions, these adjustments could have a material impact on the Company's consolidated financial statements. Significant estimates and assumptions in estimating the fair value of acquired customer relationships and other identifiable intangible assets include future cash flows that the Company expects to generate from the acquired assets. If the subsequent actual results and updated projections of the underlying business activity change compared with the assumptions and projections used to develop these values, the Company could record impairment charges. In addition, the Company has estimated the economic lives of certain acquired assets and these lives are used to calculate depreciation and amortization expense. If the Company estimates the economic lives change, depreciation or amortization expenses could be increased or decreased, or the acquired asset could be impaired. For each acquisition, the Company allocated the purchase price to the assets acquired and the liabilities assumed based on their fair values as of their respective acquisition dates. The results of operations for the acquired companies were included in the Company’s consolidated statements of income and comprehensive income beginning on their respective acquisition dates.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying amounts reflected in the consolidated balance sheets for cash and cash equivalents, trade accounts receivable, trade accounts payable, and borrowings under the line of credit and term loan approximate their respective fair values based on the short-term nature of these instruments. At December 31, 2017 and 2016, the fair value of fixed rate debt was approximately $1,497,179,000 and $549,000,000, respectively. The fair value of fixed rate debt is designated as Level 2 in the fair value hierarchy (i.e., significant observable inputs) and is based primarily on the discounted value of future cash flows using current market interest rates offered for debt of similar credit risk and maturity.
Non-derivative Financial Instrument Designated as a Net Investment Hedge	Non-derivative Financial Instrument Designated as a Net Investment Hedge The Company designated euro-denominated debt, a non-derivative financial instrument, as a hedge against a portion of the Company's euro-denominated net investment in its European subsidiaries. Changes in the value of the euro-denominated debt attributable to the change in exchange rates at the end of each reporting period are expected to offset the foreign currency translation adjustments resulting from the euro-denominated net investment, and are reported as a component of accumulated other comprehensive loss on the Company's consolidated balance sheet. The net investment hedge is discussed further in the non-derivative financial instrument footnote.
Shipping and Handling Costs	Shipping and Handling Costs Shipping and handling costs are classified as selling, administrative and other expenses in the accompanying consolidated statements of income and comprehensive income and totaled approximately $290,000,000, $230,000,000, and $240,000,000, for the years ended December 31, 2017, 2016, and 2015, respectively.
Advertising Costs	Advertising Costs Advertising costs are expensed as incurred and totaled $64,700,000, $66,900,000, and $75,000,000 in the years ended December 31, 2017, 2016, and 2015, respectively.
Accounting for Legal Costs	Accounting for Legal Costs The Company’s legal costs expected to be incurred in connection with loss contingencies are expensed as such costs are incurred.
Share-Based Compensation	Share-Based Compensation The Company maintains various long-term incentive plans, which provide for the granting of stock options, stock appreciation rights (SARs), restricted stock, restricted stock units (RSUs), performance awards, dividend equivalents and other share-based awards. SARs represent a right to receive upon exercise an amount, payable in shares of common stock, equal to the excess, if any, of the fair market value of the Company’s common stock on the date of exercise over the base value of the grant. The terms of such SARs require net settlement in shares of common stock and do not provide for cash settlement. RSUs represent a contingent right to receive one share of the Company’s common stock at a future date. The majority of awards previously granted vest on a pro-rata basis for periods ranging from one to five years and are expensed accordingly on a straight-line basis. The Company issues new shares upon exercise or conversion of awards under these plans.
Net Income per Common Share	Net Income per Common Share Basic net income per common share is computed by dividing net income by the weighted average number of common shares outstanding during the year. The computation of diluted net income per common share includes the dilutive effect of stock options, stock appreciation rights and nonvested restricted stock awards options. Options to purchase approximately 1,920,000, 1,290,000, and 1,280,000 shares of common stock ranging from $85 — $100 per share were outstanding at December 31, 2017, 2016, and 2015, respectively. These options were excluded from the computation of diluted net income per common share because the options’ exercise prices were greater than the average market prices of common stock in each respective year.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Revenue from Contracts with Customers (Topic 606) In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) ("ASU 2014-09"), which will create a single, comprehensive revenue recognition model for recognizing revenue from contracts with customers. The standard is effective for interim and annual reporting periods beginning after December 15, 2017. Accordingly, the Company will adopt this standard on January 1, 2018. The core principle of the new standard is that a company should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and more judgment and estimates are required within the revenue recognition process than are required under existing guidance, including identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation, among others. The Company has established a cross-functional implementation team to evaluate and implement the new standard related to the recognition of revenue from contracts with customers. The Company plans to use the modified retrospective adoption method. As a result, a cumulative effect adjustment is required at January 1, 2018 and the Company will account for revenue under the new standard prospectively from such date. The Company primarily sells goods and recognizes revenue at point of sale or delivery and this will not change under the new standard. However, certain customer relationships have terms that include items considered variable consideration, primarily related to customer discounts which will require a change in recognition under the new standard. Upon adoption of Topic 606, the cumulative impact to the Company’s retained earnings at January 1, 2018 is estimated to be approximately $8,000,000. Once finalized, this amount will be recorded as a reduction in retained earnings as a cumulative effect of adoption of a new accounting standard and a deferred revenue liability will be established and classified with accrued liabilities on the Company’s consolidated balance sheet. Leases (Topic 842) In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) ("ASU 2016-02"), which requires an entity to recognize a right-of-use asset and a lease liability on the balance sheet for all leases, including operating leases, with a term greater than twelve months. Expanded disclosures with additional qualitative and quantitative information will also be required. This guidance is effective for interim and annual reporting periods beginning after December 15, 2018 and early adoption is permitted. The new standard must be adopted using a modified retrospective transition. The Company has established a cross-functional team to evaluate and implement the new standard. As disclosed in the leased properties footnote, the future minimum payments under noncancelable operating leases are approximately $1,140,000,000 and the Company believes the adoption of this standard will have a significant impact on the consolidated balance sheet. Income Tax Reform The Tax Cuts and Jobs Act (the Act) was enacted December 22, 2017. The Act reduces the U.S. federal corporate tax rate from 35% to 21% for taxable years starting after December 31, 2017, and requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were not previously subject to U.S. Federal income tax and creates new taxes on certain foreign sourced earnings. As of December 31, 2017, the Company has not completed the accounting for the tax effects of enactment of the Act; however, the Company has made a reasonable estimate of the effect of the Act on the existing deferred tax balances and of the one-time transition tax. As disclosed in the income taxes footnote, the items for which the Company was able to determine a reasonable estimate were recognized as a provisional tax expense of $50,986,000 for the period ended December 31, 2017, which is included as a component of income tax expense in the Company's consolidated statement of income and comprehensive income. In all cases, the Company will continue to make and refine the calculations as additional analysis is completed. Further, the Company's estimates may also be affected as regulations and additional guidance are made available. In addition, the Act subjects a U.S. shareholder to tax on Global Intangible Low-Taxed Income (GILTI) earned by certain foreign subsidiaries. Given the complexity of the GILTI provisions, the Company is still evaluating the effects and has not yet determined the new accounting policy. The provision is not expected to have a material impact on the Company’s consolidated financial statements or related disclosures. Inventory (Topic 330) In July 2015, the FASB issued ASU 2015-11, Inventory (Topic 330): Simplifying the Measurement of Inventory ("ASU 2015-11"), which modifies existing requirements regarding measuring first-in, first-out and average cost inventory at the lower of cost or market. Under existing standards, the market amount requires consideration of replacement cost, net realizable value (“NRV”), and NRV less an approximately normal profit margin. ASU 2015-11 replaces market with NRV, defined as estimated selling prices in the ordinary course of business, less reasonably predictable costs of completion, disposal and transportation. This eliminates the need to determine and consider replacement cost or NRV less an approximately normal profit margin when measuring inventory. The Company adopted ASU 2015-11 on January 1, 2017 and it did not have a material impact to the Company's consolidated financial statements. Compensation—Stock Compensation (Topic 718) In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting ("ASU 2016-09") that changes the accounting for certain aspects of share-based compensation to employees including forfeitures, employer tax withholding, and the financial statement presentation of excess tax benefits or expense. ASU 2016-09 also clarifies the statement of cash flows presentation for certain components of share-based compensation, which prospectively reclassifies cash flows from excess tax benefits of share-based compensation currently disclosed in financing activities to operating activities in the period of adoption. The guidance will increase income tax expense volatility, as well as the Company's cash flows from operations. In addition, the Company did not elect to change shares withheld for employment income tax purposes. The Company adopted ASU 2016-09 on January 1, 2017 on a prospective basis. The adoption of ASU 2016-09 did not have a material impact to the Company's consolidated financial statements or related disclosures. Compensation-Retirement Benefits (Topic 715) In March 2017, the FASB issued ASU 2017-07, Compensation-Retirement Benefits (Topic 715) ("ASU 2017-07"), which requires an entity to report the service cost component of net periodic benefit cost in the same line item as other compensation costs (selling, administrative and other expenses), and the remaining components in non-operating expense in the consolidated statement of income and comprehensive income. This standard is effective for interim and annual reporting periods beginning after December 15, 2017 and early adoption is permitted. The Company will adopt ASU 2017-07 on January 1, 2018 and it is not expected to have a material impact on the Company’s consolidated financial statements or related disclosures.
Share-based Compensation, Option and Incentive Plans	For the years ended December 31, 2017, 2016, and 2015, the fair values for SARs granted were estimated using a Black-Scholes option pricing model with the following weighted-average assumptions, respectively: risk-free interest rate of 2.3%, 1.6%, and 2.0%; dividend yield of 2.8%, 2.7%, and 2.6%; annual historical volatility factor of the expected market price of the Company’s common stock of 19% for each of the three years and an average expected life of approximately six years. The fair value of RSUs is based on the price of the Company’s stock on the date of grant.
Pension and Other Postretirement Plans	The Company’s benefit plan committees in the U.S. and Canada establish investment policies and strategies and regularly monitor the performance of the funds. The plans in Europe are unfunded and, therefore, there are no plan assets. The pension plan strategy implemented by the Company’s management is to achieve long-term objectives and invest the pension assets in accordance with the applicable pension legislation in the U.S. and Canada as well as fiduciary standards. The long-term primary investment objectives for the pension plans are to provide for a reasonable amount of long-term growth of capital, without undue exposure to risk, protect the assets from erosion of purchasing power, and provide investment results that meet or exceed the pension plans’ actuarially assumed long-term rates of return. The Company’s investment strategy with respect to pension plan assets is to generate a return in excess of the passive portfolio benchmark (47% S&P 500 Index, 5% Russell Mid Cap Index, 7% Russell 2000 Index, 5% MSCI EAFE Index, 5% DJ Global Moderate Index, 3% MSCI Emerging Market Net, and 28% BarCap U.S. Govt/Credit). The fair values of the plan assets as of December 31, 2017 and 2016, by asset category, are shown in the tables below. Various inputs are considered when determining the value of the Company’s pension plan assets. The inputs or methodologies used for valuing securities are not necessarily an indication of the risk associated with investing in these securities. Level 1 represents observable market inputs that are unadjusted quoted prices for identical assets or liabilities in active markets. Level 2 represents other significant observable inputs (including quoted prices for similar securities, interest rates, credit risk, etc.). Level 3 represents significant unobservable inputs (including the Company’s own assumptions in determining the fair value of investments). Certain investments are measured at fair value using the net asset value ("NAV") per share as a practical expedient and have not been classified in the fair value hierarchy. The valuation methods may produce a fair value calculation that may not be indicative of net realizable value or reflective of future fair values. Furthermore, while the Company believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date. Equity securities are valued at the closing price reported on the active market on which the individual securities are traded on the last day of the calendar plan year. Debt securities including corporate bonds, U.S. Government securities, and asset-backed securities are valued using price evaluations reflecting the bid and/or ask sides of the market for an investment as of the last day of the calendar plan year. Based on the investment policy for the pension plans, as well as an asset study that was performed based on the Company’s asset allocations and future expectations, the Company’s expected rate of return on plan assets for measuring 2018 pension income is 7.20% for the plans. The asset study forecasted expected rates of return for the approximate duration of the Company’s benefit obligations, using capital market data and historical relationships. The Company has one defined contribution plan in the U.S. that covers substantially all of its domestic employees. Employees receive a matching contribution of 100% of the first 5% of the employees’ salary. The Company’s funding policy is to contribute an amount equal to the minimum required contribution under applicable pension legislation. For the plans in the U.S. and Canada, the Company may increase its contribution above the minimum, if appropriate to its tax and cash position and the plans’ funded position. For the plans in Europe, these plans will be funded in accordance with local regulations. The Company also sponsors supplemental retirement plans covering employees in the U.S. and Canada. The Company uses a measurement date of December 31 for its pension and supplemental retirement plans. Several assumptions are used to determine the benefit obligations, plan assets, and net periodic income. The discount rate for the pension plans is calculated using a bond matching approach to select specific bonds that would satisfy the projected benefit payments. The bond matching approach reflects the process that would be used to settle the pension obligations. The expected return on plan assets is based on a calculated market-related value of plan assets, where gains and losses on plan assets are amortized over a five year period and accumulate in other comprehensive income. Other non-investment unrecognized gains and losses are amortized in future net income based on a “corridor” approach, where the corridor is equal to 10% of the greater of the benefit obligation or the market-related value of plan assets at the beginning of the year. The unrecognized gains and losses in excess of the corridor criteria are amortized over the average future lifetime or service of plan participants, depending on the plan. These assumptions are updated at each annual measurement date.
Consolidation, Variable Interest Entity	The Company guarantees the borrowings of certain independently controlled automotive parts stores (independents) and certain other affiliates in which the Company has a noncontrolling equity ownership interest (affiliates). Presently, the independents are generally consolidated by unaffiliated enterprises that have a controlling financial interest through ownership of a majority voting interest in the independent. The Company has no voting interest or other equity conversion rights in any of the independents. The Company does not control the independents or the affiliates, but receives a fee for the guarantee. The Company has concluded that the independents are variable interest entities, but that the Company is not the primary beneficiary. Specifically, the equity holders of the independents have the power to direct the activities that most significantly impact the entity’s economic performance including, but not limited to, decisions about hiring and terminating personnel, local marketing and promotional initiatives, pricing and selling activities, credit decisions, monitoring and maintaining appropriate inventories, and store hours. Separately, the Company concluded the affiliates are not variable interest entities. The Company’s maximum exposure to loss as a result of its involvement with these independents and affiliates is generally equal to the total borrowings subject to the Company’s guarantee. While such borrowings of the independents and affiliates are outstanding, the Company is required to maintain compliance with certain covenants, including a maximum debt to EBITDA ratio and certain limitations on additional borrowings. At December 31, 2017, the Company was in compliance with all such covenants. At December 31, 2017, the total borrowings of the independents and affiliates subject to guarantee by the Company were approximately $616,710,000. These loans generally mature over periods from one to six years. In the event that the Company is required to make payments in connection with guaranteed obligations of the independents or the affiliates, the Company would obtain and liquidate certain collateral (e.g., accounts receivable and inventory) to recover all or a portion of the amounts paid under the guarantee. When it is deemed probable that the Company will incur a loss in connection with a guarantee, a liability is recorded equal to this estimated loss. To date, the Company has had no significant losses in connection with guarantees of independents’ and affiliates’ borrowings.
Segment Reporting	The Company’s reportable segments consist of automotive, industrial, business products, and electrical/electronic materials. Within the reportable segments, certain of the Company’s operating segments are aggregated since they have similar economic characteristics, products and services, type and class of customers, and distribution methods.