SECURITIES - Schedule of Securities Under Agreements to Repurchase or Resell Remaining Contractual Maturity (Details) (Mortgage-backed securities - Agency, USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ (3,400)
Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	0
Up to 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	(350)
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	(1,000)
Greater Than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ (2,050)