Debt and Equity Transactions (Unaudited) Interest, Dividends and Maturities (Details) - USD ($) shares in Millions			1 Months Ended	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Jul. 29, 2016	Jan. 05, 2016	Nov. 08, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Capitalized Interest [Line Items]
Interest costs incurred				$ 211,000,000		$ 211,000,000		$ 647,000,000	$ 622,000,000
Common Stock, Value of Shares Authorized for Issue in Transaction	$ 12,000,000			$ 12.39
Stock Issued During Period, Shares, New Issues				33.5			28.0
Proceeds from Issuance of Common Stock, Gross				$ 415,000,000			$ 311,000,000	1,500,000,000		$ 2,000,000,000
Fees and Commissions		$ 20,000,000		4,000,000
Property, Plant, Equipment and Mining Development Costs, Net [Member]
Capitalized Interest [Line Items]
Interest costs capitalized				24,000,000		42,000,000		66,000,000	134,000,000
Oil and Gas Properties Not Subject to Amortization [Member]
Capitalized Interest [Line Items]
Interest costs capitalized				$ 0	$ 0	$ 12,000,000		$ 7,000,000	$ 50,000,000
Subsequent Event [Member]
Capitalized Interest [Line Items]
Common Stock, Value of Shares Authorized for Issue in Transaction			$ 11.54
Stock Issued During Period, Shares, New Issues			26.3
Proceeds from Issuance of Common Stock, Gross			$ 304,000,000
Fees and Commissions			$ 3,000,000