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Fair Value Measurements Changes in Fair Value Level 3 (Details) (Investment Segment [Member], USD $)
In Millions, unless otherwise specified
6 Months Ended 12 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Dec. 31, 2010
Investment Segment [Member]
       
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) Included in Other Income     $ 8  
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, [Roll Forward]        
Balance at January 1 289 329 329 228
Gross realized and unrealized gains 2 2 8 18
Gross proceeds (4) (42) (48) (138)
Gross purchases     0 221
Balance at June 30 $ 287 $ 289 $ 289 $ 329