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Long-Term Debt - Schedule of Principal Amount, Effective Interest Rate and Maturity Date for Corporation's Senior Notes (Detail) - USD ($)
12 Months Ended
Dec. 20, 2017
Dec. 31, 2018
Dec. 31, 2017
May 22, 2017
4.25% Senior Notes, Due 2024        
Debt Instrument [Line Items]        
Principal Amount   $ 400,000,000    
Effective Interest Rate   4.25%    
Maturity Date   Jul. 02, 2024    
7% Debentures, Due 2025        
Debt Instrument [Line Items]        
Principal Amount   $ 125,000,000    
Effective Interest Rate   7.12%    
Maturity Date   Dec. 01, 2025    
3.450% Senior Notes, Due 2027        
Debt Instrument [Line Items]        
Principal Amount   $ 300,000,000   $ 300,000,000
Effective Interest Rate   3.47%    
Maturity Date   Jun. 01, 2027    
3.500% Senior Notes, Due 2027        
Debt Instrument [Line Items]        
Principal Amount $ 500,000,000 $ 500,000,000    
Effective Interest Rate   3.53%    
Maturity Date   Dec. 15, 2027    
6.25% Senior Notes, Due 2037        
Debt Instrument [Line Items]        
Principal Amount   $ 230,000,000    
Effective Interest Rate   6.45%    
Maturity Date   May 01, 2037    
4.250% Senior Notes, Due 2047        
Debt Instrument [Line Items]        
Principal Amount 600,000,000 $ 600,000,000    
Effective Interest Rate   4.27%    
Maturity Date   Dec. 15, 2047    
Floating Rate Notes, Due 2019        
Debt Instrument [Line Items]        
Principal Amount $ 300,000,000 $ 300,000,000    
Effective Interest Rate   3.29% 2.13%  
Debt Instrument Effective Interest Rate   Three-month LIBOR + 0.50%    
Maturity Date Dec. 20, 2019 Dec. 20, 2019    
Floating Rate Notes, Due 2020        
Debt Instrument [Line Items]        
Principal Amount   $ 300,000,000   $ 300,000,000
Effective Interest Rate   3.30% 2.10%  
Debt Instrument Effective Interest Rate   Three-month LIBOR + 0.65%    
Maturity Date   May 22, 2020