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Retirement Benefits - Fair Value Recurring Basis (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Fair Value Defined Benefit Plan Measured On Recurring Basis Financial Statement Captions [Line Items]    
Cash equivalents $ 383 $ 995 [1]
[1] Excludes certain cash equivalents considered to be held-to-maturity and not reported at fair value.