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Fair Value Measurements Unobservable Inputs Rollforward (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 5 $ 6
Return of principal on ARS (3) (1)
Included in investment income 3  
Included in other comprehensive income (4)  
Ending balance $ 0 $ 5