Pension and Other Postretirement Benefits - Reconciliation of Fair Value Measurements Using Significant Unobservable Inputs (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 2,603	$ 2,484
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	160	174
Transfer in (out) of Level 3		(3)
Purchases	4	4
Sales	(8)	(19)
Settlements	(15)	(12)
Unrealized gain (loss)	(8)	7
Realized gain	15	9
Fair value of plan assets at end of year	148	160
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	35	31
Sales	(3)	(5)
Unrealized gain (loss)	2	7
Realized gain	2	2
Fair value of plan assets at end of year	36	35
Significant Unobservable Inputs (Level 3) [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year		13
Transfer in (out) of Level 3		(3)
Sales		(1)
Settlements		2
Fair value of plan assets at end of year	11	11
Significant Unobservable Inputs (Level 3) [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	53	56
Purchases	3	2
Settlements	(6)	(4)
Unrealized gain (loss)	(12)	(7)
Realized gain	9	6
Fair value of plan assets at end of year	47	53
Significant Unobservable Inputs (Level 3) [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	61	74
Purchases	1	2
Sales	(5)	(13)
Settlements	(9)	(10)
Unrealized gain (loss)	2	7
Realized gain	4	1
Fair value of plan assets at end of year	$ 54	$ 61