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Debt and Other Financing Arrangements - Schedule of Debt and Other Financing Arrangements (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Total long-term debt $ 3,760,600 $ 4,360,600
Less debt issuance costs 18,358 21,459
Total amounts outstanding 3,742,242 4,339,141
Less current maturities 500,000 600,000
Total long-term debt due after one year 3,242,242 3,739,141
Notes, 5.75%, Due 2017 [Member]    
Debt Instrument [Line Items]    
Debt instrument face amount   600,000
Notes, 5.85%, Due 2018 [Member]    
Debt Instrument [Line Items]    
Debt instrument face amount 500,000 500,000
Notes, 4.125%, Due 2022 [Member]    
Debt Instrument [Line Items]    
Debt instrument face amount 600,000 600,000
Notes, 4.0%, Due 2023 [Member]    
Debt Instrument [Line Items]    
Debt instrument face amount 500,000 500,000
Notes, 6.40%, Due 2037 [Member]    
Debt Instrument [Line Items]    
Debt instrument face amount 650,000 650,000
Notes, 5.20%, Due 2043 [Member]    
Debt Instrument [Line Items]    
Debt instrument face amount 500,000 500,000
Industrial Revenue Bonds [Member] | Due from 2020 to 2040 [Member]    
Debt Instrument [Line Items]    
Debt instrument face amount $ 1,010,600 $ 1,010,600