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Long-term debt - Narrative (Detail)
Jun. 21, 2018
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]    
Shelf registration statement maximum amount of securities authorized for issuance   $ 7,000,000,000
Contractual debt maturities, 2019   420,000,000
Contractual debt maturities, 2020   725,000,000
Contractual debt maturities, 2021   350,000,000
Line of Credit | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 1,510,000,000.00  
Revolving Credit Facility    
Debt Instrument [Line Items]    
Unused borrowing capacity   $ 1,440,000,000
Debt Covenant, Period One | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Covenant requirement, maximum total leverage ratio 5.0  
Debt Covenant, Period Two | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Covenant requirement, maximum total leverage ratio 4.75  
Debt Covenant, Period Three | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Covenant requirement, maximum total leverage ratio 4.5