COMMITMENTS, GUARANTEES, PRODUCT WARRANTIES AND OTHER LOSS CONTINGENCIES - WMC Legal Matters (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) lawsuit securitization	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Claims reflecting purchase price or unpaid principal balances of the loans at the time of purchase	$ 3,660
Period prior to notifying WMC when claim would be disallowed	6 years
Disposed of by Sale | WMC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Active claims for alleged breaches of representations and warranties on mortgage loans	$ 462	$ 1,060
Additional claims asserted against WMC in litigation without making a prior claim	3,198	5,456
Reserves related to repurchase claims	416	$ 626	$ 875
Net decrease to reserves due to settlements	$ 210
Number of lawsuits arising from alleged breaches of representations and warranties | lawsuit	5
Number of securitizations related to lawsuits | securitization	6
Disposed of by Sale | WMC | Pending Outcome of Ongoing Settlement Negotiations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of lawsuits arising from alleged breaches of representations and warranties | lawsuit	4
Disposed of by Sale | WMC | Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Estimate of possible loss	$ 0
Disposed of by Sale | WMC | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Estimate of possible loss	$ 500