Fair Value Measurement - FV of Fin. Instruments (Details 2) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Carrying Amount [Member]
Financial Assets [Abstract]
Investment in Cenovus Energy	$ 2,084	$ 0
Commodity derivatives, assets	80	91
Total loans and advances - related parties	583	701
Financial Liabilities [Abstract]
Total debt, excluding capital leases, carrying amount	20,181	26,423
Commodity derivatives, liabilities	63	101
Investment in Cenovus Energy	2,084	0
Commodity derivatives, assets	84	91
Total loans and advances - related parties	583	701
Total debt, excluding capital leases, carrying amount	23,451	29,307
Commodity derivatives, liabilities	$ 72	$ 113