Benefit Plans - Plan Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Balance at January 1	$ 471	$ 500
Actual Return on Assets Still Held at December 31	29	40
Actual Return on Assets Sold During the Year Ended December 31	(1)	1
Net Purchases, Sales, and Settlements	(24)	(70)
Net Transfers In (Out) of Level 3	(5)
Balance at December 31	470	471
Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance at January 1	15	15
Net Transfers In (Out) of Level 3	(5)
Balance at December 31	10	15
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance at January 1		8
Net Purchases, Sales, and Settlements		(8)
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance at January 1	333	352
Actual Return on Assets Still Held at December 31	19	21
Net Purchases, Sales, and Settlements	(25)	(40)
Balance at December 31	327	333
Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance at January 1	123	125
Actual Return on Assets Still Held at December 31	10	19
Actual Return on Assets Sold During the Year Ended December 31	(1)	1
Net Purchases, Sales, and Settlements	1	(22)
Balance at December 31	133	123
Limited Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance at January 1	456	477
Actual Return on Assets Still Held at December 31	29	40
Actual Return on Assets Sold During the Year Ended December 31	(1)	1
Net Purchases, Sales, and Settlements	(24)	(62)
Balance at December 31	$ 460	$ 456