Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance				$ (313)	$ (204)
Other comprehensive loss before reclassifications				(6)	(82)
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit	[1]			(38)	(38)
Amortization - actuarial loss	[1]			45	40
Amortization - settlement loss	[1]			4	90
Tax effect				(4)	(34)
Other comprehensive income (loss)		$ (3)	$ (10)	1	(24)
Ending balance		(312)	(228)	(312)	(228)
Accumulated Defined Benefit Plans Adjustment | Pension Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance				(217)	(161)
Other comprehensive loss before reclassifications				(17)	(88)
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit	[1]			(35)	(35)
Amortization - actuarial loss	[1]			39	38
Amortization - settlement loss	[1]			4	90
Tax effect				(3)	(35)
Other comprehensive income (loss)				(12)	(30)
Ending balance		(229)	(191)	(229)	(191)
Accumulated Defined Benefit Plans Adjustment | Other Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance				(104)	(50)
Other comprehensive loss before reclassifications				14	8
Amounts reclassified from accumulated other comprehensive loss:
Amortization - prior service credit	[1]			(3)	(3)
Amortization - actuarial loss	[1]			6	2
Amortization - settlement loss				0	0
Tax effect				(1)	1
Other comprehensive income (loss)				16	8
Ending balance		(88)	(42)	(88)	(42)
Gain on Cash Flow Hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance				4	4
Other comprehensive loss before reclassifications				0	0
Amounts reclassified from accumulated other comprehensive loss:
Tax effect				0	0
Other comprehensive income (loss)				0	0
Ending balance		4	4	4	4
Workers Compensation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance				4	3
Other comprehensive loss before reclassifications				(3)	(2)
Amounts reclassified from accumulated other comprehensive loss:
Tax effect				0	0
Other comprehensive income (loss)				(3)	(2)
Ending balance		$ 1	$ 1	$ 1	$ 1

[1] These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost. See Note 19.