Other Comprehensive Income - Reclassifications (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification from accumulated other comprehensive loss
Amortization of pension and other postretirement benefits	$ 5			$ 1			$ 12	$ 3
Total before tax	(36)			(9)			(45)	3
Income tax expense	(3)			(8)			(3)	0
Net (loss) income	(39)	$ (17)	$ 8	(17)	$ 22	$ (2)	(48)	3
Reclassification out of Accumulated Other Comprehensive Income
Reclassification from accumulated other comprehensive loss
Income tax expense	0			0			0	0
Net (loss) income	3			4			10	12
Reclassification out of Accumulated Other Comprehensive Income | Pension and other postretirement benefits adjustments net of tax
Reclassification from accumulated other comprehensive loss
Total before tax	3			4			10	12
Reclassification out of Accumulated Other Comprehensive Income | Actuarial loss
Reclassification from accumulated other comprehensive loss
Amortization of pension and other postretirement benefits	3			3			10	11
Reclassification out of Accumulated Other Comprehensive Income | Prior service credit
Reclassification from accumulated other comprehensive loss
Amortization of pension and other postretirement benefits	$ 0			$ 1			$ 0	$ 1