Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss by Component) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (512)	$ (320)
Other comprehensive income (loss), before reclassifications, net of tax		(2)	(4)
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit		(10)	(11)
Amortization - actuarial loss		14	9
Amortization - settlement loss		0	0
Other		(1)	(1)
Tax effect		0	1
Other comprehensive income (loss)		1	(6)
Ending balance		(511)	(326)
Other comprehensive income (loss) before reclassifications, tax		0	(1)
Accumulated Defined Benefit Plans Adjustment | Pension Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(338)	(212)
Other comprehensive income (loss), before reclassifications, net of tax		(2)	(2)
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit	[1]	(11)	(11)
Amortization - actuarial loss	[1]	12	8
Amortization - settlement loss	[1]	0	0
Tax effect		(1)	1
Other comprehensive income (loss)		(2)	(4)
Ending balance		(340)	(216)
Accumulated Defined Benefit Plans Adjustment | Other Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(181)	(116)
Other comprehensive income (loss), before reclassifications, net of tax		0	(2)
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit	[1]	1	0
Amortization - actuarial loss	[1]	2	1
Amortization - settlement loss		0	0
Tax effect		0	0
Other comprehensive income (loss)		3	(1)
Ending balance		(178)	(117)
Gain on Cash Flow Hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		1	1
Other comprehensive income (loss), before reclassifications, net of tax		0	0
Amounts reclassified from accumulated other comprehensive loss:
Tax effect		0	0
Other comprehensive income (loss)		0	0
Ending balance		1	1
Workers Compensation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		6	7
Other comprehensive income (loss), before reclassifications, net of tax		0	0
Amounts reclassified from accumulated other comprehensive loss:
Other		(1)	(1)
Tax effect		1	0
Other comprehensive income (loss)		0	(1)
Ending balance		$ 6	$ 6

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