Plan Assets Related to Other Postretirement Benefits by Level within the Fair Value Hierarchy (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Plan assets	$ 160	[1],[2]	$ 158	[1],[2]	$ 164	[1]
Fair Value, Inputs, Level 1 [Member]
Plan assets	160	[2]	158	[2]
Fair Value, Inputs, Level 2 [Member]
Plan assets	0	[2]	0	[2]
Fair Value, Inputs, Level 3 [Member]
Plan assets	0	[2]	0	[2]
Equity Funds [Member]
Plan assets	51	[2]	52	[2]
Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Plan assets	51	[2]	52	[2]
Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Plan assets	0	[2]	0	[2]
Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Plan assets	0	[2]	0	[2]
Nontaxable Municipal Bonds [Member]
Plan assets	82	[2]	85	[2]
Nontaxable Municipal Bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Plan assets	82	[2]	85	[2]
Nontaxable Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Plan assets	0	[2]	0	[2]
Nontaxable Municipal Bonds [Member] | Fair Value, Inputs, Level 3 [Member]
Plan assets	0	[2]	0	[2]
Taxable Fixed Income Index Funds [Member]
Plan assets	14	[2]	14	[2]
Taxable Fixed Income Index Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Plan assets	14	[2]	14	[2]
Taxable Fixed Income Index Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Plan assets	0	[2]	0	[2]
Taxable Fixed Income Index Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Plan assets	0	[2]	0	[2]
Money Market Funds [Member]
Plan assets	13	[2]	7	[2]
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Plan assets	13	[2]	7	[2]
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Plan assets	0	[2]	0	[2]
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Plan assets	$ 0	[2]	$ 0	[2]

[1] 2 Plan remeasured at December 31, 2013 due to plan amendment.

[2] 1 Fair value measurements do not include accrued income.