Financial Instruments (Narrative) (Details) $ in Millions, $ in Billions			1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 CAD ($)	Mar. 31, 2019 CAD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2019 CAD ($)
Derivative [Line Items]
Loss on Cronos-related financial instruments					$ 140	$ 1,442	$ 0
Cronos [Member]
Derivative [Line Items]
Payments to acquire equity method investments and derivative instruments, investing	$ 1,800	$ 2.4	$ 2.4
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative [Line Items]
Derivative asset, notional amount	$ 1,800			$ 1,800				$ 2.4
Loss on Cronos-related financial instruments				$ 31