Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (320)	$ (144)
Other comprehensive income (loss), before reclassifications, net of tax		(4)	(1)
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit		(11)	(11)
Amortization - actuarial loss		9	4
Amortization - settlement loss		0	0
Other		(1)	(1)
Tax effect		1	2
Other comprehensive loss		(6)	(7)
Ending balance		(326)	(151)
Other comprehensive income (loss) before reclassifications, tax		1	0
Accumulated Defined Benefit Plans Adjustment | Pension Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(212)	(132)
Other comprehensive income (loss), before reclassifications, net of tax		(2)	(1)
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit	[1]	(11)	(11)
Amortization - actuarial loss	[1]	8	4
Amortization - settlement loss	[1]	0	0
Tax effect		1	2
Other comprehensive loss		(4)	(6)
Ending balance		(216)	(138)
Accumulated Defined Benefit Plans Adjustment | Other Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(116)	(23)
Other comprehensive income (loss), before reclassifications, net of tax		(2)	0
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit	[1]	0	0
Amortization - actuarial loss	[1]	1	0
Amortization - settlement loss		0	0
Tax effect		0	0
Other comprehensive loss		(1)	0
Ending balance		(117)	(23)
Gain on Cash Flow Hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		1	2
Other comprehensive income (loss), before reclassifications, net of tax		0	0
Amounts reclassified from accumulated other comprehensive loss:
Tax effect		0	0
Other comprehensive loss		0	0
Ending balance		1	2
Workers Compensation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		7	9
Other comprehensive income (loss), before reclassifications, net of tax		0	0
Amounts reclassified from accumulated other comprehensive loss:
Other		(1)	(1)
Tax effect		0	0
Other comprehensive loss		(1)	(1)
Ending balance		$ 6	$ 8

[1] These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost. See Note 21.