CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (1,232)	$ (5,269)	$ (1,733)
Items related to employee benefit plans:
Change in net actuarial (loss) gain, net of, $26, $60, and $(45) tax	(92)	(195)	133
Change in net prior service cost, net of $(12), $(4), and $(3) tax	33	13	9
Curtailment loss, net of $(1), $—, and $— tax	3	0	0
Reclassification of realized loss on interest rate swaps to net income, net of $(16), $—, and $— tax	46	2
Reclassification of realized loss on interest rate swaps to net income, net of $(16), $—, and $— tax			0
Unrealized holding loss on interest rate swaps, net of $29, $12, and $— tax	(86)	(41)
Unrealized holding loss on interest rate swaps, net of $29, $12, and $— tax			0
Foreign currency translation adjustment, net of $(43), $(6), and $50 tax	(37)	2	(201)
Other comprehensive loss	(133)	(219)	(59)
COMPREHENSIVE LOSS	$ (1,365)	$ (5,488)	$ (1,792)