Derivative Instruments and Hedging Activities - Incremental Collateral and Termination Payments upon Potential Future Ratings Downgrade (Details) $ in Millions	Mar. 31, 2021 USD ($)
Bilateral Downgrade Agreement
Derivatives
Incremental collateral or termination payments upon potential future ratings downgrade	$ 336
One-notch downgrade
Derivatives
Incremental collateral or termination payments upon potential future ratings downgrade	231
Two-notch downgrade
Derivatives
Incremental collateral or termination payments upon potential future ratings downgrade	$ 242