SHAREHOLDERS' EQUITY - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended				12 Months Ended
		Jan. 31, 2018	Mar. 31, 2018		Mar. 31, 2017		Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other than temporary impairment losses realized			$ (7)		$ (10)
Total before tax			982		898
Income tax (expense) or benefit			(265)		(306)
Net of tax			$ 717		$ 592
Blended statutory income tax rate			27.00%
Statutory U.S. tax rate		21.00%	21.00%		35.00%		35.00%
Reclassification Out Of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax			$ (2)		$ (12)
Reclassification Out Of Accumulated Other Comprehensive Income | Unrealized gains (losses) on investment securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Sales and redemptions			2		(6)
Other than temporary impairment losses realized			0		(10)
Total before tax			2		(16)
Income tax (expense) or benefit			(1)	[1]	6	[2]
Net of tax			1		(10)
Reclassification Out Of Accumulated Other Comprehensive Income | Pension liability adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Acquisition and operating expenses, actuarial gains (losses)	[3]		(4)		(3)
Acquisition and operating expenses, prior service (cost) credit	[3]		0		0
Income tax (expense) or benefit			1	[1]	1	[2]
Net of tax			$ (3)		$ (2)

[1] Based on 27% blended tax rate

[2] Based on 35% tax rate

[3] These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see Note 12 for additional details)