Indebtedness (Narrative) (Details)	12 Months Ended
	Dec. 31, 2016 USD ($)	Jan. 27, 2017 USD ($)	Jan. 27, 2016 USD ($)	Dec. 31, 2015 USD ($)
Indebtedness [Line Items]
Committed credit facilities, maximum borrowing capacity	$ 8,000,000,000
Ratio of consolidated EBITDA to consolidated interest expense on a rolling 4 quarter basis, minimum (not less than)	3.5
Ratio of consolidated EBITDA to consolidated interest expense on a rolling 4 quarter basis	10.6
Amount Outstanding	$ 643,000,000			$ 825,000,000
Bank Loans [Member]
Indebtedness [Line Items]
Amount Outstanding	643,000,000			825,000,000
Subsidiaries [Member] | Bank Loans [Member]
Indebtedness [Line Items]
Amount Outstanding	643,000,000			825,000,000
364-day revolving credit, expiring February 7, 2017 [Member]
Indebtedness [Line Items]
Committed credit facilities, maximum borrowing capacity	2,000,000,000		$ 2,000,000,000
Multi-year revolving credit, expiring February 28, 2021 [Member]
Indebtedness [Line Items]
Committed credit facilities, maximum borrowing capacity	2,500,000,000		$ 2,500,000,000
364-day revolving credit, expiring February 2018 [Member] | Subsequent Event [Member]
Indebtedness [Line Items]
Committed credit facilities, maximum borrowing capacity		$ 2,000,000,000.0
Short-term credit arrangement [Member] | Subsidiaries [Member]
Indebtedness [Line Items]
Committed credit facilities, maximum borrowing capacity	$ 2,900,000,000			$ 2,900,000,000