Plan Assets Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	$ 185	$ 181
Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	106	105
Equity Securities [Member] | Growth Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	50	53
Equity Securities [Member] | Value Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	52	52
Equity Securities [Member] | Blended Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	4
Mutual And Exchange Traded Funds [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	42	39
Mutual And Exchange Traded Funds [Member] | Money Market Funds
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	4	5
Mutual And Exchange Traded Funds [Member] | International Fund [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	29	25
Mutual And Exchange Traded Funds [Member] | Commodities Investment [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	9	9
Debt Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	37	37
Debt Securities [Member] | US Treasury Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	13	10
Debt Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	21	25
Debt Securities [Member] | Non Agency Residential Mortgage Backed Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	2	1
Debt Securities [Member] | Corporate Bond Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	1	1
Fair Value, Inputs, Level 1
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	161	154
Fair Value, Inputs, Level 1 | Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	106	105
Fair Value, Inputs, Level 1 | Equity Securities [Member] | Growth Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	50	53
Fair Value, Inputs, Level 1 | Equity Securities [Member] | Value Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	52	52
Fair Value, Inputs, Level 1 | Equity Securities [Member] | Blended Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	4
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	42	39
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds [Member] | Money Market Funds
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	4	5
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds [Member] | International Fund [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	29	25
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds [Member] | Commodities Investment [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	9	9
Fair Value, Inputs, Level 1 | Debt Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	13	10
Fair Value, Inputs, Level 1 | Debt Securities [Member] | US Treasury Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	13	10
Fair Value, Inputs, Level 2
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	24	27
Fair Value, Inputs, Level 2 | Debt Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	24	27
Fair Value, Inputs, Level 2 | Debt Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	21	25
Fair Value, Inputs, Level 2 | Debt Securities [Member] | Non Agency Residential Mortgage Backed Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	2	1
Fair Value, Inputs, Level 2 | Debt Securities [Member] | Corporate Bond Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	$ 1	$ 1