Derivative Instruments - Additional Information (Details) € in Millions, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2021 CAD ($)	Sep. 30, 2021 EUR (€)	Dec. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Net unrealized loss recognized in AOCI	$ (13,000,000)	$ 17,000,000	$ (68,000,000)	$ 268,000,000
Interest Rate Swaps - Beginning October 31. 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional value	3,500,000,000		3,500,000,000
Interest Rate Swaps - Beginning September 30, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional value	500,000,000		500,000,000
Interest rate swaps
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional value	3,500,000,000		3,500,000,000		$ 3,500,000,000
Net unrealized loss recognized in AOCI			143,000,000		213,000,000
Loss reclassified from AOCI to income			28,000,000		50,000,000
Cross currency interest rate contract | Fixed income interest rate
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional value	5,000,000,000		5,000,000,000			$ 6,754
Cross currency interest rate contract | Fixed income interest rate | Derivatives designated as net investment hedges
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional value					$ 500,000,000			$ 400,000,000
Cross currency interest rate contract | Fixed income interest rate | Hedge Funds
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional value	1,200,000,000		1,200,000,000				€ 1,108
Foreign Exchange Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional value	$ 148,000,000		$ 148,000,000