Employee Benefit Plans - Fair Value of Plan Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	$ 2,043,379	$ 2,206,479	$ 1,965,502
Assets Measured at NAV	252,488	285,876
Common Stocks - Mutual Funds - Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	457,567	536,609
Assets Measured at NAV	166,045	193,628
Genuine Parts Company Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	193,810	191,771
Assets Measured at NAV	0	0
Other Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	713,924	838,694
Assets Measured at NAV	0	0
Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	30,855	47,745
Assets Measured at NAV	0	0
Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	14,583	13,530
Assets Measured at NAV	0	0
Government Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	223,750	180,838
Assets Measured at NAV	0	0
Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	227,616	207,978
Assets Measured at NAV	0	0
Asset Backed and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	8,866	9,725
Assets Measured at NAV	0	0
Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		211
Assets Measured at NAV		0
Other-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	29,471	29,431
Assets Measured at NAV	0	0
Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	8,747	7,346
Assets Measured at NAV	0	0
Municipal Funds-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	131,755	139,801
Assets Measured at NAV	86,443	92,248
Options and Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		38
Assets Measured at NAV		0
Cash Surrender Value of Life Insurance Policies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	2,435	2,762
Assets Measured at NAV	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	1,433,261	1,585,779
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Common Stocks - Mutual Funds - Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	291,522	342,981
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Genuine Parts Company Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	193,810	191,771
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	713,882	838,659
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	30,855	47,745
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	14,583	13,530
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Government Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	159,483	121,834
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Asset Backed and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	29,126	29,221
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Municipal Funds-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Options and Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		38
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash Surrender Value of Life Insurance Policies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		0
Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	355,153	332,027
Significant Observable Inputs (Level 2) [Member] | Common Stocks - Mutual Funds - Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Observable Inputs (Level 2) [Member] | Genuine Parts Company Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Observable Inputs (Level 2) [Member] | Other Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Observable Inputs (Level 2) [Member] | Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Observable Inputs (Level 2) [Member] | Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Observable Inputs (Level 2) [Member] | Government Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	64,267	59,004
Significant Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	227,616	207,978
Significant Observable Inputs (Level 2) [Member] | Asset Backed and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	8,866	9,725
Significant Observable Inputs (Level 2) [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		211
Significant Observable Inputs (Level 2) [Member] | Other-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	345	210
Significant Observable Inputs (Level 2) [Member] | Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	8,747	7,346
Significant Observable Inputs (Level 2) [Member] | Municipal Funds-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	45,312	47,553
Significant Observable Inputs (Level 2) [Member] | Options and Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		0
Significant Observable Inputs (Level 2) [Member] | Cash Surrender Value of Life Insurance Policies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	2,477	2,797
Significant Unobservable Inputs (Level 3) [Member] | Common Stocks - Mutual Funds - Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Genuine Parts Company Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Other Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	42	35
Significant Unobservable Inputs (Level 3) [Member] | Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Government Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Asset Backed and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		0
Significant Unobservable Inputs (Level 3) [Member] | Other-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Municipal Funds-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Options and Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets		0
Significant Unobservable Inputs (Level 3) [Member] | Cash Surrender Value of Life Insurance Policies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	$ 2,435	$ 2,762