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Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Derivative [Line Items]    
Pre-tax net unrealized gain (loss) $ 5,726,740 $ 5,168,361
Unrealized gain (loss) on hedging instruments    
Derivative [Line Items]    
Pre-tax net unrealized gain (loss) $ 1,400