Bank Loans and Related Allowance for Credit Losses - Composition of Bank Loans and Delinquency Analysis by Loan Segment (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Financing Receivable, Past Due [Line Items]
Total loans	$ 40,815,000,000	$ 40,477,000,000
Allowance for credit losses	32,000,000	38,000,000	$ 74,000,000	$ 73,000,000
Total bank loans – net	40,783,000,000	40,439,000,000
Loans accruing interest contractually 90 days or more past due	0	0
Current
Financing Receivable, Past Due [Line Items]
Total loans	40,758,000,000	40,414,000,000
Total past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	57,000,000	63,000,000
30-59 days past due
Financing Receivable, Past Due [Line Items]
Total loans	34,000,000	45,000,000
60-89 days past due
Financing Receivable, Past Due [Line Items]
Total loans	8,000,000	3,000,000
More than or equal to 90 days past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	15,000,000	15,000,000
Residential real estate
Financing Receivable, Past Due [Line Items]
Total loans	26,656,000,000	26,632,000,000
Allowance for credit losses	28,000,000	34,000,000	71,000,000	70,000,000
Total bank loans – net	26,628,000,000	26,598,000,000
Unamortized premiums and discounts and direct origination costs	$ 101,000,000	$ 100,000,000
Residential real estate | Loans Geographic Area | California | Geographic Concentration Risk
Financing Receivable, Past Due [Line Items]
Concentration risk percentage	42.00%	43.00%
Residential real estate | Current
Financing Receivable, Past Due [Line Items]
Total loans	$ 26,607,000,000	$ 26,584,000,000
Residential real estate | Total past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	49,000,000	48,000,000
Residential real estate | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Total loans	30,000,000	34,000,000
Residential real estate | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Total loans	4,000,000	3,000,000
Residential real estate | More than or equal to 90 days past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	15,000,000	11,000,000
Residential real estate | First Mortgages
Financing Receivable, Past Due [Line Items]
Total loans	26,201,000,000	26,153,000,000
Allowance for credit losses	27,000,000	32,000,000	67,000,000	66,000,000
Total bank loans – net	26,174,000,000	26,121,000,000
Residential real estate | First Mortgages | Current
Financing Receivable, Past Due [Line Items]
Total loans	26,157,000,000	26,111,000,000
Residential real estate | First Mortgages | Total past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	44,000,000	42,000,000
Residential real estate | First Mortgages | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Total loans	29,000,000	33,000,000
Residential real estate | First Mortgages | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Total loans	4,000,000	2,000,000
Residential real estate | First Mortgages | More than or equal to 90 days past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	11,000,000	7,000,000
Residential real estate | HELOCs
Financing Receivable, Past Due [Line Items]
Total loans	455,000,000	479,000,000
Allowance for credit losses	1,000,000	2,000,000	4,000,000	4,000,000
Total bank loans – net	454,000,000	477,000,000
Residential real estate | HELOCs | Current
Financing Receivable, Past Due [Line Items]
Total loans	450,000,000	473,000,000
Residential real estate | HELOCs | Total past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	5,000,000	6,000,000
Residential real estate | HELOCs | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Total loans	1,000,000	1,000,000
Residential real estate | HELOCs | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Total loans	0	1,000,000
Residential real estate | HELOCs | More than or equal to 90 days past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	4,000,000	4,000,000
Pledged asset lines
Financing Receivable, Past Due [Line Items]
Total loans	13,841,000,000	13,548,000,000
Allowance for credit losses	0	0	0	0
Total bank loans – net	13,841,000,000	13,548,000,000
Pledged asset lines | Current
Financing Receivable, Past Due [Line Items]
Total loans	13,833,000,000	13,533,000,000
Pledged asset lines | Total past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	8,000,000	15,000,000
Pledged asset lines | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Total loans	4,000,000	11,000,000
Pledged asset lines | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Total loans	4,000,000	0
Pledged asset lines | More than or equal to 90 days past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	0	4,000,000
Other
Financing Receivable, Past Due [Line Items]
Total loans	318,000,000	297,000,000
Allowance for credit losses	4,000,000	4,000,000	$ 3,000,000	$ 3,000,000
Total bank loans – net	314,000,000	293,000,000
Other | Current
Financing Receivable, Past Due [Line Items]
Total loans	318,000,000	297,000,000
Other | Total past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	0	0
Other | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Total loans	0	0
Other | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Total loans	0	0
Other | More than or equal to 90 days past due and other nonaccrual loans
Financing Receivable, Past Due [Line Items]
Total loans	$ 0	$ 0