DISCONTINUED OPERATIONS - Components of the Net Gain on WRECO Divestiture (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Shares tendered		0	0	58,813,000
WRECO
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common shares tendered (58,813,151 shares at $33.22 per share)				$ 1,954
Cash	$ 707			707
Proceeds				2,661
Net book value of contributed assets				(1,671)
Transaction costs, net of reimbursement				(18)
Net book value of contributed assets and transaction costs, net of reimbursement				(1,689)
Net gain on divestitures		$ 0	$ 0	$ 972
Shares tendered	59,000,000			58,813,151
Price of shares tendered				$ 33.22