MORTGAGE BANKING - Changes Related to MSRs - Fair Value Method (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
		Sep. 30, 2018	Sep. 30, 2018
MSRs
Fair value at end of the period		$ 612	$ 612
Residential mortgages
MSRs
Fair value as of beginning of period		0	0
Acquired MSRs		590	590
Amounts capitalized		29	29
Changes in unpaid principal balance during the period	[1]	(12)	(12)
Changes in fair value during the period	[2]	5	5
Fair value at end of the period		$ 612	$ 612

[1] Represents changes in value due to i) passage of time including the impact from both regularly scheduled loan principal payments and partial paydowns, and ii) loans that paid off during the period.

[2] Represents changes in value primarily due to market driven changes in interest rates and prepayment speeds.