Derivative Financial Instruments - Summary of the volume and fair value positions of derivative instruments (Details) $ in Millions	Sep. 30, 2021 USD ($) contract	Dec. 31, 2020 USD ($) contract
Derivatives, Fair Value
Total asset derivatives, notional amount	$ 1,673	$ 2,009
Total liability derivatives, notional amount	3,639	3,182
Total derivatives, notional amount	$ 5,312	$ 5,191
Total asset derivatives, number of contracts | contract	4,491	4,440
Total liability derivatives, number of contracts | contract	7,984	4,108
Total derivatives, Number of contracts | contract	12,475	8,548
Net amount on balance sheet	$ 148	$ 206
Net amount on balance sheet	(567)	(655)
Total derivatives, fair value, net	(419)	(449)
Asset derivatives gross amount	150	218
Liability derivatives, gross asset	22	2
Asset derivatives, gross liability	(2)	(12)
Liability derivatives gross amount	(589)	(657)
Liabilities held for sale
Derivatives, Fair Value
Total liability derivatives, notional amount	$ 2,146	$ 2,240
Total liability derivatives, number of contracts | contract	2,554	2,737
Net amount on balance sheet	$ (549)	$ (630)
Liability derivatives, gross asset	3	1
Liability derivatives gross amount	(552)	(631)
Assets held for sale
Derivatives, Fair Value
Total asset derivatives, notional amount	$ 3	$ 158
Total asset derivatives, number of contracts | contract	2,561	3,189
Net amount on balance sheet	$ 126	$ 185
Asset derivatives gross amount	126	189
Asset derivatives, gross liability	$ 0	$ (4)
Futures | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	479	290
Net amount on balance sheet	$ 0	$ 0
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	$ 0	$ 0
Futures | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, number of contracts | contract	4,019	705
Net amount on balance sheet	$ (1)	$ 0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	$ (1)	$ 0
Options | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	61	56
Net amount on balance sheet	$ 4	$ 6
Asset derivatives gross amount	4	6
Asset derivatives, gross liability	$ 0	$ 0
Futures | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	1,390	905
Net amount on balance sheet	$ 4	$ 1
Asset derivatives gross amount	4	1
Asset derivatives, gross liability	$ 0	$ 0
Futures | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, number of contracts | contract	1,411	666
Net amount on balance sheet	$ (3)	$ 0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(3)	0
Total return swap agreements – fixed income | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount		50
Net amount on balance sheet		0
Liability derivatives, gross asset		0
Liability derivatives gross amount		0
Foreign currency forwards | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	137	291
Net amount on balance sheet	1	4
Asset derivatives gross amount	2	9
Asset derivatives, gross liability	(1)	(5)
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	607	250
Net amount on balance sheet	14	(9)
Liability derivatives, gross asset	19	1
Liability derivatives gross amount	(5)	(10)
Other embedded derivative financial instruments | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	750	750
Net amount on balance sheet	0	0
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	0	0
Credit default swaps – buying protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	33	60
Net amount on balance sheet	(1)	(3)
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	(1)	(3)
Credit default swaps – buying protection | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	881	638
Net amount on balance sheet	(28)	(16)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(28)	(16)
Credit default swaps – selling protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	750	750
Net amount on balance sheet	14	13
Asset derivatives gross amount	14	13
Asset derivatives, gross liability	0	0
Credit default swaps – selling protection | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	5	4
Net amount on balance sheet	0	0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	$ 0	$ 0