Hedging Activities - Narrative (Details) $ in Millions	3 Months Ended				9 Months Ended			12 Months Ended
	Dec. 31, 2024 CAD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2024 CAD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2024 USD ($)	Jun. 30, 2024 CAD ($)	Mar. 31, 2024 CAD ($)	Jun. 15, 2023 USD ($)
Derivative [Line Items]
Other comprehensive income (loss), cash flows hedge, gain (loss) reclassification, before tax		$ 0	$ 0			$ 0	$ 0
Net Investment Hedging | CAD | Currency swap | Derivatives designated for hedge accounting
Derivative [Line Items]
Derivative, notional amount, entered into during period	$ 6,000				$ 2,000
Derivative, notional amount	3,500				3,500					$ 2,500	$ 1,500
Derivative notional amount, terminated	$ 5,000				$ 5,000					$ 1,500
Fair Value Hedges | Floating interest rate swaps | Derivatives designated for hedge accounting
Derivative [Line Items]
Derivative, notional amount								$ 1,300,000,000	$ 1,300,000,000
Derivative notional amount, terminated									$ 500,000,000
Cash Flow Hedges | Currency swap | Derivatives designated for hedge accounting
Derivative [Line Items]
Other comprehensive income (loss), cash flows hedge, gain (loss) reclassification, before tax		$ 16,000,000	$ 9,000,000			$ 2,000,000	$ 36,000,000
Cash Flow Hedges | Floating interest rate swaps | Derivatives designated for hedge accounting
Derivative [Line Items]
Derivative, notional amount, entered into during period				$ 50,000,000				$ 450,000,000
Derivative, notional amount												$ 500,000,000
Derivative termination proceeds, amortization period (in years)					10 years	10 years