Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	$ 3,638		$ 3,412	[1]
Restricted Cash and Investments, Current	853	[1]	850		738
Fuel derivative contracts, Fair Value	66	[1]	65		97
Fair value of assets and liabilities measured on recurring basis	4,557		4,327
Cash Flow Hedge Derivative Instrument Assets at Fair Value			65
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	417		438	[1]
Restricted Cash and Investments, Current	853	[1]	850	[1]
Fuel derivative contracts, Fair Value	0	[1]
Fair value of assets and liabilities measured on recurring basis	1,270		1,288
Cash Flow Hedge Derivative Instrument Assets at Fair Value			0	[1]
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	3,221		2,974	[1]
Restricted Cash and Investments, Current	0	[1]	0	[1]
Fuel derivative contracts, Fair Value	66	[1]
Fair value of assets and liabilities measured on recurring basis	3,287		3,039
Cash Flow Hedge Derivative Instrument Assets at Fair Value			65	[1]
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0		0	[1]
Restricted Cash and Investments, Current	0	[1]	0	[1]
Fuel derivative contracts, Fair Value	0	[1]
Fair value of assets and liabilities measured on recurring basis	0		0
Cash Flow Hedge Derivative Instrument Assets at Fair Value			0	[1]
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	417	[1],[2]	438	[1],[3]
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	417	[1],[2]	438	[1],[3]
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0	[1],[2]	0	[1],[3]
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0	[1],[2]	0	[1],[3]
US Government Agencies Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	609		545	[1],[3]
US Government Agencies Debt Securities | Maturity Dates Exceeding One Year
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	609		545
US Government Agencies Debt Securities | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0		0	[1],[3]
US Government Agencies Debt Securities | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	609		545	[1],[3]
US Government Agencies Debt Securities | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0		0	[1],[3]
Repurchase Agreements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	280		304	[1],[3]
Repurchase Agreements [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0		0	[1],[3]
Repurchase Agreements [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	280		304	[1],[3]
Repurchase Agreements [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0		0	[1],[3]
Corporate Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	1,749		1,519	[1],[3]
Corporate Obligations | Maturity Dates Exceeding One Year
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	470		405
Corporate Obligations | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0			[1],[3]
Corporate Obligations | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	1,749		1,519	[1],[3]
Corporate Obligations | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0		0	[1],[3]
Bank Time Deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	583		606	[1],[3]
Bank Time Deposits | Maturity Dates Exceeding One Year
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	350		200
Bank Time Deposits | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	0			[1],[3]
Bank Time Deposits | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	583		606	[1],[3]
Bank Time Deposits | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	$ 0		$ 0	[1],[3]

[1] Unrealized gains or losses on short-term investments, restricted cash and short-term investments and derivatives qualifying for hedge accounting are recorded in Accumulated other comprehensive income (loss) at each measurement date.

[2] The Company's short-term investments mature in one year or less except for $350 million of Bank notes/Certificates of deposit/Time deposits, $609 million of U.S. Government agency investments and $470 million of Corporate obligations which have maturity dates exceeding one year.

[3] The Company's short-term investments mature in one year or less except for $200 million of Bank notes/Certificates of deposit/Time deposits, $545 million of U.S. Government agency investments and $405 million of Corporate obligations which have maturity dates exceeding one year.