RECLASSIFICATIONS FROM OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassifications from accumulated other comprehensive (loss) income
Reclassification from AOCI, Current Period, Tax	$ 59
Income from cash and cash investments and short-term bank deposits		$ (5)	$ (4)	$ (4)
Income tax expense		258	304	296
Other nonoperating income (expense)		(13)	(32)	(26)
Net income available to common shareholders		727	528	487
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
Reclassifications from accumulated other comprehensive (loss) income
Reclassification from accumulated other comprehensive income, before tax		20	86	19
Reclassification from AOCI, Current Period, Tax		(5)	(13)	(7)
Reclassification from accumulated other comprehensive income, net of tax		15	73	12
Accumulated Other Comprehensive Income (Loss) - Interest rate swaps | Amount reclassified
Reclassifications from accumulated other comprehensive (loss) income
Income tax expense		0	(1)	(1)
Net income available to common shareholders		2	3	1
Accumulated Other Comprehensive Income (Loss) - Available -for-Sale Securities | Amount reclassified
Reclassifications from accumulated other comprehensive (loss) income
Income tax expense		0	0	(15)
Other nonoperating income (expense)		0	0	(3)
Net income available to common shareholders		0	0	12
Federal
Reclassifications from accumulated other comprehensive (loss) income
Disproportionate Tax Effect in OCI				14
Interest Rate Swaps | Accumulated Other Comprehensive Income (Loss) - Interest rate swaps | Amount reclassified
Reclassifications from accumulated other comprehensive (loss) income
Income from cash and cash investments and short-term bank deposits		$ 2	$ 4	$ 2