Capital Stock (Schedule of Shares Repurchased) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended		18 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2019
Class of Stock [Line Items]
Total number of shares repurchased (in shares)	16,503,317	27,878,324
Aggregate cost of shares repurchased	$ 845	$ 1,673
Average price per share of shares repurchased (USD per share)	$ 51.24	$ 60.00
July 2019 Share Repurchase Program [Member]
Class of Stock [Line Items]
Total number of shares repurchased (in shares)	10,100,000
Aggregate cost of shares repurchased	$ 500
Average price per share of shares repurchased (USD per share)	$ 49.29
January 2018 Share Repurchase Program [Member]
Class of Stock [Line Items]
Total number of shares repurchased (in shares)	6,400,000	27,600,000	34,000,000.0
Aggregate cost of shares repurchased	$ 345	$ 1,655
Average price per share of shares repurchased (USD per share)	$ 54.36	$ 59.89	$ 58.86
July 2015 Share Repurchase Program [Member]
Class of Stock [Line Items]
Total number of shares repurchased (in shares)		300,000
Aggregate cost of shares repurchased		$ 18
Average price per share of shares repurchased (USD per share)		$ 71.68