Derivatives - Fair Value Hedge (Details) - USD ($)	Sep. 30, 2021	Mar. 31, 2021
Floating rate registered note due 2049
Derivatives
Principal amount	$ 500,000,000
Floating rate registered note due 2049 | LIBOR
Derivatives
Principal amount	500,000,000
Floating rate registered note due 2050 | LIBOR
Derivatives
Principal amount	300,000,000
Fixed rate registered note due 2049
Derivatives
Principal amount		$ 1,000,000,000
Fixed rate registered note due 2050
Derivatives
Principal amount		$ 650,000,000
Interest rate contracts
Derivatives
Derivative notional amount	$ 800,000,000