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Derivative and Financial Instruments - FX Notional (Details)
£ in Millions, $ in Millions
 6 Months Ended
Jun. 30, 2019
CAD ($)
Jun. 30, 2019
GBP (£)
Jun. 30, 2019
USD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2018
GBP (£)
Dec. 31, 2018
USD ($)
Trading Activity, Gains and Losses, Net [Line Items]            
Description of Foreign Currency Derivative Instruments Not Designated as Hedging Instruments Activities In December 2017, we entered into foreign exchange zero cost collars buying the right to sell $1.25 billion CAD at $0.707 CAD and selling the right to buy $1.25 billion CAD at $0.842 CAD against the U.S. dollar. The collar expired during the second quarter of 2019 and we entered into new foreign currency exchange forward contracts to sell $1.35 billion CAD at $0.748 CAD against the U.S. dollar.          
Sell U S Dollar Buy British pound [Member]            
Trading Activity, Gains and Losses, Net [Line Items]            
Net notional position of foreign currency exchange derivatives | $     $ 0     $ 805,000,000
Sell British pound, buy other currencies [Member]            
Trading Activity, Gains and Losses, Net [Line Items]            
Net notional position of foreign currency exchange derivatives | £   £ 0     £ 21  
Sell Canadian dollar, buy U.S. dollar [Member]            
Trading Activity, Gains and Losses, Net [Line Items]            
Net notional position of foreign currency exchange derivatives | $ $ 1,350     $ 1,242    
Buy British Pound Sell Euro [Member]            
Trading Activity, Gains and Losses, Net [Line Items]            
Net notional position of foreign currency exchange derivatives | £   £ 15     £ 0