Fair Value of Pension Plan Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 4,501	$ 3,698
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	443	166
U.S Government and Government Agency Issues
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	457	347
Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	850	632
Common Stock Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	545	880
Common Stock Mid Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	371	359
Common Stock Small Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	94	111
Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	1,010	1,350
Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	336	328
Common/Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	900	565
Partnership Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	388	209
Other Holdings
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	117	101
Collateral Held on Loaned Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	126	208
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	1,130	1,438
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	16	32
Fair Value, Inputs, Level 1 | Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	13
Fair Value, Inputs, Level 1 | Common Stock Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	545	880
Fair Value, Inputs, Level 1 | Common Stock Mid Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	371	359
Fair Value, Inputs, Level 1 | Common Stock Small Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	94	111
Fair Value, Inputs, Level 1 | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	1,010	1,350
Fair Value, Inputs, Level 1 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	118	112
Fair Value, Inputs, Level 1 | Other Holdings
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	10	4
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	2,216	1,615
Fair Value, Inputs, Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	427	134
Fair Value, Inputs, Level 2 | U.S Government and Government Agency Issues
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	457	347
Fair Value, Inputs, Level 2 | Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	837	632
Fair Value, Inputs, Level 2 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	218	216
Fair Value, Inputs, Level 2 | Common/Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	143	131
Fair Value, Inputs, Level 2 | Other Holdings
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	97	95
Fair Value, Inputs, Level 2 | Collateral Held on Loaned Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	126	208
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	16	8	$ 8
Fair Value, Inputs, Level 3 | Common/Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	6	6	6
Fair Value, Inputs, Level 3 | Other Holdings
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	10	2	$ 2
Fair Value, Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	1,139	637
Fair Value, Measured at NAV | Common/Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	751	428
Fair Value, Measured at NAV | Partnership Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	388	209
Securities Pledged as Collateral
Defined Benefit Plan Disclosure [Line Items]
Collateral to be paid on loaned securities	(126)	(208)
Securities Pledged as Collateral | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Collateral to be paid on loaned securities	(37)	(60)
Securities Pledged as Collateral | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Collateral to be paid on loaned securities	$ (89)	$ (148)