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Derivatives - Cash Flow Hedges (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2019
USD ($)
Bcf
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
Bcf
Jun. 30, 2018
USD ($)
Dec. 31, 2018
Bcf
Derivative [Line Items]          
Natural gas held in storage (in cubic feet) | Bcf 4   4   5
Forecast sale of natural gas from storage designated as cash-flow hedges (in cubic feet) | Bcf         4
Unrealized loss included in other comprehensive income [1] $ 18,000,000 $ 1,000,000 $ 16,000,000 $ 4,000,000  
Cash Flow Hedging | Interest Rate Contract, 10-Year Tenor          
Derivative [Line Items]          
Treasury tenor     10 years    
Notional value $ 750,000,000   $ 750,000,000    
Weighted Average Fixed Rate 2.11%   2.11%    
Unrealized loss included in other comprehensive income     $ 7,000,000    
Liability $ 7,000,000   $ 7,000,000    
Cash Flow Hedging | Interest Rate Contract, 30-Year Tenor          
Derivative [Line Items]          
Treasury tenor     30 years    
Notional value $ 750,000,000   $ 750,000,000    
Weighted Average Fixed Rate 2.59%   2.59%    
Unrealized loss included in other comprehensive income     $ 11,000,000    
Liability $ 11,000,000   11,000,000    
Cash Flow Hedging | Interest Rate Contract | Level 2          
Derivative [Line Items]          
Liability $ 18,000,000   $ 18,000,000    
[1]
Net of tax of $5 and zero for the three months ended June 30, 2019, and 2018, and $5 and $1 for the six months ended June 30, 2019, and 2018, respectively.