Pension and Other Postretirement Benefits - Schedule of Changes in Benefit Obligations and Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Change in Benefit Obligation
Service cost	$ 57	$ 44	$ 53
Interest cost	109	109	100
Change in Plan Assets
Fair value of plan assets at beginning of year	1,557
Fair value of plan assets at end of year	1,397	1,557
Pension Benefits [Member] | Investments Measured at Fair Value [Member]
Change in Benefit Obligation
Benefit obligation at beginning of year	2,638	2,238
Service cost	57	44
Interest cost	109	109
Actuarial loss (gain)	(151)	401
Benefits paid	(163)	(154)
Benefit obligation at end of year	2,490	2,638	2,238
Change in Plan Assets
Fair value of plan assets at beginning of year	2,236	2,116
Actual return on plan assets	(61)	268
Company and participant contributions	6	6
Benefits paid by plan	(163)	(154)
Fair value of plan assets at end of year	2,018	2,236	2,116
Funded Status at end of year (plan assets less plan obligations)	(472)	(402)
Other Postretirement Benefits [Member]
Change in Benefit Obligation
Service cost	7	7	8
Interest cost	24	26	29
Change in Plan Assets
Fair value of plan assets at beginning of year	340
Fair value of plan assets at end of year	326	340
Other Postretirement Benefits [Member] | Investments Measured at Fair Value [Member]
Change in Benefit Obligation
Benefit obligation at beginning of year	632	574
Service cost	7	7
Interest cost	24	26
Actuarial loss (gain)	(61)	59
Benefits paid	(39)	(34)
Benefit obligation at end of year	563	632	574
Change in Plan Assets
Fair value of plan assets at beginning of year	367	368
Actual return on plan assets	1	21
Company and participant contributions	5	6
Benefits paid by plan	(25)	(28)
Fair value of plan assets at end of year	348	367	$ 368
Funded Status at end of year (plan assets less plan obligations)	$ (215)	$ (265)