Pension and Other Postretirement Benefits - Schedule of Changes in Benefit Obligations and Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in Plan Assets
Fair value of plan assets at end of year	$ 2,603	$ 2,484
Pension Benefits [Member]
Change in Benefit Obligation
Benefit obligation at beginning of year	2,238	2,494
Service cost	44	53	35
Interest cost	109	100	107
Amendments		3
Actuarial loss (gain)	401	(277)
Benefits paid	(154)	(135)
Benefit obligation at end of year	2,638	2,238	2,494
Change in Plan Assets
Fair value of plan assets at beginning of year	2,116	2,039
Actual return on plan assets	268	86
Company and participant contributions	6	126
Benefits paid	(154)	(135)
Fair value of plan assets at end of year	2,236	2,116	2,039
Funded Status at end of year (plan assets less plan obligations)	(402)	(122)
Other Postretirement Benefits [Member]
Change in Benefit Obligation
Benefit obligation at beginning of year	574	775
Service cost	7	8	7
Interest cost	26	29	35
Amendments		(124)
Actuarial loss (gain)	59	(71)
Benefits paid	(34)	(43)
Benefit obligation at end of year	632	574	775
Change in Plan Assets
Fair value of plan assets at beginning of year	368	321
Actual return on plan assets	21	56
Company and participant contributions	12	34
Benefits paid	(34)	(43)
Fair value of plan assets at end of year	367	368	321
Funded Status at end of year (plan assets less plan obligations)	$ (265)	$ (206)