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	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other gains (losses)											$ (104)		$ (349)		$ (114)
Total before tax			$ 1,058	$ 1,036	$ 1,109	$ 1,242	$ 722	$ 1,146	$ 1,131	$ 982	4,445		3,983		4,018
Income tax (expense) benefit			$ (276)	$ (259)	$ (292)	$ (314)	$ (197)	$ (301)	$ (299)	$ (265)	(1,141)		(1,063)		586
Net of tax											$ 3,304		$ 2,920		$ 4,604
Blended statutory income tax rate											26.00%		27.00%
U.S. federal statutory corporate income tax rate		21.00%									21.00%		21.00%		35.00%
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax											$ 2		$ (47)		$ (12)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized gains (losses) on investment securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses realized											(13)		(63)
Other-than-temporary impairment losses realized															(29)
Other gains (losses)											31		17		27
Total before tax											18		(46)		(2)
Income tax (expense) benefit											(5)	[1]	12	[2]	1	[3]
Net of tax											13		(34)		(1)
Reclassification out of Accumulated Other Comprehensive Income | Pension liability adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Acquisition and operating expense, actuarial gains (losses)	[4]										(15)		(18)		(17)
Acquisition and operating expenses, prior service (cost) credit	[4]										0		0		0
Income tax (expense) benefit											4	[1]	5	[2]	6	[3]
Net of tax											$ (11)		$ (13)		$ (11)

[1] Based on 26% blended tax rate

[2] Based on 27% blended tax rate

[3] Based on 35% tax rate

[4] These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see Note 14 for additional details).