SECURITIES - Schedule of Securities Under Agreements to Repurchase or Resell Remaining Contractual Maturity (Details) $ in Millions	Sep. 30, 2015 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	$ 1,300
Securities sold under agreements to repurchase	(1,800)
Mortgage-backed securities - Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	1,300
Securities sold under agreements to repurchase	(1,800)
Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	0
Securities sold under agreements to repurchase	0
Overnight and Continuous | Mortgage-backed securities - Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	0
Securities sold under agreements to repurchase	0
Up to 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	500
Securities sold under agreements to repurchase	(500)
Up to 30 Days | Mortgage-backed securities - Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	500
Securities sold under agreements to repurchase	(500)
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	300
Securities sold under agreements to repurchase	(800)
30-90 Days | Mortgage-backed securities - Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	300
Securities sold under agreements to repurchase	(800)
Greater Than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	500
Securities sold under agreements to repurchase	(500)
Greater Than 90 Days | Mortgage-backed securities - Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities purchased under agreements to resell	500
Securities sold under agreements to repurchase	$ (500)