Bank Loans and Related Allowance for Credit Losses - Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 38	$ 73	$ 73
Charge-offs	0	0
Recoveries	0	0
Provision for credit losses	(6)	1
Balance at end of period	32	74	38
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	34	70	70
Charge-offs	0	0
Recoveries	0	0
Provision for credit losses	(6)	1
Balance at end of period	28	71	34
Residential real estate | First Mortgages
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	32	66	66
Charge-offs	0	0	0
Recoveries	0	0
Provision for credit losses	(5)	1
Balance at end of period	27	67	32
Residential real estate | HELOCs
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	2	4	4
Charge-offs	0	0
Recoveries	0	0
Provision for credit losses	(1)	0
Balance at end of period	1	4	2
Pledged asset lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	0	0	0
Charge-offs	0	0
Recoveries	0	0
Provision for credit losses	0	0
Balance at end of period	0	0	0
Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	4	3	3
Charge-offs	0	0
Recoveries	0	0
Provision for credit losses	0	0
Balance at end of period	$ 4	$ 3	$ 4