SHAREHOLDERS' INTEREST - Changes in amounts included in our cumulative other comprehensive income (loss) by component (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Beginning balance	$ (1,562)	$ (1,562)	$ (1,459)
Other comprehensive income (loss) before reclassifications (1)		347	(234)
Amounts reclassified from accumulated other comprehensive income (loss) to earnings(1)(2)		325	131
Total other comprehensive income (loss)		672	(103)
Reclassification of certain tax effects, due to tax law changes		(253)
Reclassification of accumulated unrealized gains on available-for-sale securities	(9)	(9)
Net amounts reclassified from accumulated other comprehensive income to retained earnings		(262)
Ending balance		(1,152)	(1,562)
Foreign currency translation adjustments
Beginning balance	264	264	232
Other comprehensive income (loss) before reclassifications (1)		(54)	32
Amounts reclassified from accumulated other comprehensive income (loss) to earnings(1)(2)		0	0
Total other comprehensive income (loss)		(54)	32
Reclassification of certain tax effects, due to tax law changes		0
Reclassification of accumulated unrealized gains on available-for-sale securities		0
Net amounts reclassified from accumulated other comprehensive income to retained earnings		0
Ending balance		210	264
Unrealized gains on available-for-sale securities
Beginning balance	9	9	7
Other comprehensive income (loss) before reclassifications (1)		0	2
Amounts reclassified from accumulated other comprehensive income (loss) to earnings(1)(2)		0	0
Total other comprehensive income (loss)		0	2
Reclassification of certain tax effects, due to tax law changes		0
Reclassification of accumulated unrealized gains on available-for-sale securities		(9)
Net amounts reclassified from accumulated other comprehensive income to retained earnings		(9)
Ending balance		0	9
Pension | Actuarial loss
Beginning balance	(1,802)	(1,802)	(1,651)
Other comprehensive income (loss) before reclassifications (1)		393	(280)
Amounts reclassified from accumulated other comprehensive income (loss) to earnings(1)(2)		322	129
Total other comprehensive income (loss)		715	(151)
Reclassification of certain tax effects, due to tax law changes		245
Reclassification of accumulated unrealized gains on available-for-sale securities		0
Net amounts reclassified from accumulated other comprehensive income to retained earnings		(245)
Ending balance		(1,332)	(1,802)
Pension | Prior service credits (costs)
Beginning balance	(8)	(8)	(9)
Other comprehensive income (loss) before reclassifications (1)		(5)	(2)
Amounts reclassified from accumulated other comprehensive income (loss) to earnings(1)(2)		3	3
Total other comprehensive income (loss)		(2)	1
Reclassification of certain tax effects, due to tax law changes		(1)
Reclassification of accumulated unrealized gains on available-for-sale securities		0
Net amounts reclassified from accumulated other comprehensive income to retained earnings		(1)
Ending balance		(11)	(8)
Other Postretirement Benefits | Actuarial loss
Beginning balance	(48)	(48)	(67)
Other comprehensive income (loss) before reclassifications (1)		12	14
Amounts reclassified from accumulated other comprehensive income (loss) to earnings(1)(2)		6	5
Total other comprehensive income (loss)		18	19
Reclassification of certain tax effects, due to tax law changes		(12)
Reclassification of accumulated unrealized gains on available-for-sale securities		0
Net amounts reclassified from accumulated other comprehensive income to retained earnings		(12)
Ending balance		(42)	(48)
Other Postretirement Benefits | Prior service credits (costs)
Beginning balance	$ 23	23	29
Other comprehensive income (loss) before reclassifications (1)		1	0
Amounts reclassified from accumulated other comprehensive income (loss) to earnings(1)(2)		(6)	(6)
Total other comprehensive income (loss)		(5)	(6)
Reclassification of certain tax effects, due to tax law changes		5
Reclassification of accumulated unrealized gains on available-for-sale securities		0
Net amounts reclassified from accumulated other comprehensive income to retained earnings		5
Ending balance		$ 23	$ 23