Plan Assets Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	$ 181	$ 197
Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	105	111
Equity Securities [Member] | Growth Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	53	58
Equity Securities [Member] | Value Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	52	53
Mutual And Exchange Traded Funds [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	39	47
Mutual And Exchange Traded Funds [Member] | Money Market Funds
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	5	6
Mutual And Exchange Traded Funds [Member] | International Fund [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	25	30
Mutual And Exchange Traded Funds [Member] | Commodities Investment [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	9	11
Debt Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	37	39
Debt Securities [Member] | US Treasury Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	10	7
Debt Securities [Member] | US Government Agencies Debt Securities
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities		1
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	25	24
Debt Securities [Member] | Non Agency Residential Mortgage Backed Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	1	6
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	154	165
Fair Value, Inputs, Level 1 | Equity Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	105	111
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	53	58
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	53
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	39	47
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	5	6
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	25	30
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	11
Fair Value, Inputs, Level 1 | Debt Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	10	7
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	10	7
Fair Value, Inputs, Level 2
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	27	32
Fair Value, Inputs, Level 2 | Debt Securities [Member]
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities	27	32
Fair Value, Inputs, Level 2 | Debt Securities [Member] | US Government Agencies Debt Securities
Schedule Of Pension And Other Postretirment Plan Assets By Fair Value [Line Items]
Securities		1
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	25	24
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	1	6
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