Reconciliation of Change in Projected Benefit Obligation and Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Balance at end of year	$ 312	$ 293	$ 256
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation:	4,068	3,383
Benefit obligation at beginning of year	4,000	4,526
Service cost	98	121	99
Interest cost	201	191	184
Actuarial (gain) loss	807	(464)
Gross benefits paid and settlements	(281)	(269)
Curtailments		(84)
Other	(22)	(21)
Benefit obligation at end of year	4,803	4,000	4,526
Balance at beginning of year	2,397	2,157
Actual return on plan assets	151	239
Employer contributions	307	277
Gross benefits paid and settlements	(281)	(269)
Other	(12)	(7)
Balance at end of year	2,562	2,397	2,157
Funded status-Net amount recognized	(2,241)	(1,603)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	1,819	2,743
Service cost	19	52	50
Interest cost	88	110	124
Plan participants' contributions	67	67
Actuarial (gain) loss	262	(640)
Federal subsidy	4	7
Plan amendments	(3)	(331)
Curtailments	8	8
Gross benefits paid	(212)	(197)
Benefit obligation at end of year	2,052	1,819	2,743
Balance at beginning of year	57	58
Actual return on plan assets	1	1
Employer contributions	144	128
Plan participants' contributions	67	67
Benefits paid	(212)	(197)
Balance at end of year	57	57	58
Funded status-Net amount recognized	$ (1,995)	$ (1,762)