Employee Benefit Plans - Fair Value of Plan Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	$ 2,206,479	$ 1,965,502	$ 1,912,736
Assets Measured at NAV	285,876	196,576
Common Stocks - Mutual Funds - Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	536,609	384,103
Assets Measured at NAV	193,628	114,182
Genuine Parts Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	191,771	192,841
Assets Measured at NAV	0	0
Other Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	838,694	793,101
Assets Measured at NAV	0	0
Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	47,745	55,607
Assets Measured at NAV	0	0
Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	13,530	15,995
Assets Measured at NAV	0	0
Government Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	180,838	157,303
Assets Measured at NAV	0	0
Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	207,978	192,457
Assets Measured at NAV	0	0
Asset Backed and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	9,725	8,872
Assets Measured at NAV	0	0
Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	211	216
Assets Measured at NAV	0	0
Other-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	29,431	24,613
Assets Measured at NAV	0	0
Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	7,346	9,272
Assets Measured at NAV	0	0
Municipal Funds-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	139,801	128,367
Assets Measured at NAV	92,248	82,394
Derivative [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	2,762	2,755
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	1,585,779	1,450,707
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	342,981	269,921
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Genuine Parts Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	191,771	192,841
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	838,659	793,007
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	47,745	55,607
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	13,530	15,995
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Government Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	121,834	102,468
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	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	29,221	20,868
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] | Derivative [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	0
Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	332,027	315,370
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 [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	59,004	54,835
Significant Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	207,978	192,457
Significant Observable Inputs (Level 2) [Member] | Asset Backed and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	9,725	8,872
Significant Observable Inputs (Level 2) [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	211	216
Significant Observable Inputs (Level 2) [Member] | Other-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	210	3,745
Significant Observable Inputs (Level 2) [Member] | Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	7,346	9,272
Significant Observable Inputs (Level 2) [Member] | Municipal Funds-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of assets	47,553	45,973
Significant Observable Inputs (Level 2) [Member] | Derivative [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	2,797	2,849
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 [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	35	94
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	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] | Derivative [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	$ 2,762	$ 2,755