Derivative Financial Instruments - Summary of the volume and fair value positions of derivative instruments (Details) $ in Millions	Jun. 30, 2021 USD ($) contract	Dec. 31, 2020 USD ($) contract
Derivatives, Fair Value
Total asset derivatives, notional amount	$ 2,580	$ 2,009
Total liability derivatives, notional amount	2,291	3,182
Total derivatives, notional amount	$ 4,871	$ 5,191
Total asset derivatives, number of contracts | contract	19,125	4,440
Total liability derivatives, number of contracts | contract	19,291	4,108
Total derivatives, Number of contracts | contract	38,416	8,548
Net amount on balance sheet	$ 228	$ 206
Net amount on balance sheet	(624)	(655)
Total derivatives, fair value, net	(396)	(449)
Asset derivatives gross amount	243	218
Liability derivatives, gross asset	2	2
Asset derivatives, gross liability	(15)	(12)
Liability derivatives gross amount	(626)	(657)
Liabilities held for sale
Derivatives, Fair Value
Total liability derivatives, notional amount	$ 2,179	$ 2,240
Total liability derivatives, number of contracts | contract	2,525	2,737
Net amount on balance sheet	$ (601)	$ (630)
Liability derivatives, gross asset	2	1
Liability derivatives gross amount	(603)	(631)
Assets held for sale
Derivatives, Fair Value
Total asset derivatives, notional amount	$ 221	$ 158
Total asset derivatives, number of contracts | contract	2,601	3,189
Net amount on balance sheet	$ 180	$ 185
Asset derivatives gross amount	182	189
Asset derivatives, gross liability	$ (2)	$ (4)
Futures | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	280	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	1,073	705
Net amount on balance sheet	$ 0	$ 0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	$ 0	$ 0
Options | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	15,061	56
Net amount on balance sheet	$ 25	$ 6
Asset derivatives gross amount	25	6
Asset derivatives, gross liability	$ 0	$ 0
Options | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, number of contracts | contract	15,000
Net amount on balance sheet	$ (13)
Liability derivatives, gross asset	0
Liability derivatives gross amount	$ (13)
Futures | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	1,183	905
Net amount on balance sheet	$ 0	$ 1
Asset derivatives gross amount	0	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	693	666
Net amount on balance sheet	$ 0	$ 0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	0	0
Total return swap agreements – fixed income | Other investments
Derivatives, Fair Value
Total asset derivatives, notional amount	50
Net amount on balance sheet	1
Asset derivatives gross amount	1
Asset derivatives, gross liability	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	593	291
Net amount on balance sheet	6	4
Asset derivatives gross amount	16	9
Asset derivatives, gross liability	(10)	(5)
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	10	250
Net amount on balance sheet	0	(9)
Liability derivatives, gross asset	0	1
Liability derivatives gross amount	0	(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	61	60
Net amount on balance sheet	(3)	(3)
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	(3)	(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	102	638
Net amount on balance sheet	(10)	(16)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(10)	(16)
Credit default swaps – selling protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	905	750
Net amount on balance sheet	19	13
Asset derivatives gross amount	19	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		4
Net amount on balance sheet		0
Liability derivatives, gross asset		0
Liability derivatives gross amount		$ 0