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Summary of Significant Accounting Principles - Significant Accounting Principles: Collateral (Details) - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Accounting Policies [Abstract]    
Securities held as collateral, fair value $ 812.4 $ 693.0
Securities received as collateral, amount re-pledged and sold $ 758.5 $ 593.8