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Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2021
Mar. 31, 2020
Sep. 30, 2020
Derivative [Line Items]          
Pre-tax net losses associated with interest rate hedges $ 4,000,000   $ 4,000,000    
Pre-tax losses associated with net investment hedges 31,000,000 $ 11,000,000 (1,000,000) $ (9,000,000)  
IRPAs          
Derivative [Line Items]          
Notional amount $ 0 $ 0 $ 0 $ 0 $ 0
Foreign Exchange Forward          
Derivative [Line Items]          
Derivative notional amount percent of required need coverage 90.00%   90.00%