Bank Loans and Related Allowance for Credit Losses (Composition of Bank Loans and Delinquency Analysis by Loan Segment) (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Current	$ 22,243,000,000	$ 18,173,000,000
Past due and other nonaccrual loans	73,000,000	57,000,000
Total loans	22,316,000,000	18,230,000,000
Allowance for credit losses	30,000,000	18,000,000	$ 30,000,000	$ 17,000,000	$ 19,000,000	$ 21,000,000
Total bank loans – net	22,286,000,000	18,212,000,000
Loans 90 days past due and still accruing	0	0
30-59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	24,000,000	30,000,000
60-89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	7,000,000	5,000,000
More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	42,000,000	22,000,000
Residential Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Current	15,192,000,000	12,770,000,000
Past due and other nonaccrual loans	62,000,000	51,000,000
Total loans	15,254,000,000	12,821,000,000
Allowance for credit losses	27,000,000	15,000,000	26,000,000	14,000,000	17,000,000	19,000,000
Total bank loans – net	15,227,000,000	12,806,000,000
Residential Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	20,000,000	26,000,000
Residential Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	1,000,000	5,000,000
Residential Real Estate [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	41,000,000	20,000,000
First Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total loans	14,340,000,000	11,704,000,000
First Mortgage [Member] | Residential Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Current	14,294,000,000	11,665,000,000
Past due and other nonaccrual loans	46,000,000	39,000,000
Total loans	14,340,000,000	11,704,000,000
Allowance for credit losses	21,000,000	11,000,000	22,000,000	10,000,000	12,000,000	14,000,000
Total bank loans – net	14,319,000,000	11,693,000,000
First Mortgage [Member] | Residential Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	19,000,000	24,000,000
First Mortgage [Member] | Residential Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	1,000,000	4,000,000
First Mortgage [Member] | Residential Real Estate [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	26,000,000	11,000,000
HELOCs [Member]
Financing Receivable, Past Due [Line Items]
Total loans	914,000,000	1,117,000,000
HELOCs [Member] | Residential Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Current	898,000,000	1,105,000,000
Past due and other nonaccrual loans	16,000,000	12,000,000
Total loans	914,000,000	1,117,000,000
Allowance for credit losses	6,000,000	4,000,000	4,000,000	4,000,000	5,000,000	5,000,000
Total bank loans – net	908,000,000	1,113,000,000
HELOCs [Member] | Residential Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	1,000,000	2,000,000
HELOCs [Member] | Residential Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	0	1,000,000
HELOCs [Member] | Residential Real Estate [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	15,000,000	9,000,000
Pledged asset lines [Member]
Financing Receivable, Past Due [Line Items]
Current	6,875,000,000	5,202,000,000
Past due and other nonaccrual loans	10,000,000	4,000,000
Total loans	6,885,000,000	5,206,000,000
Allowance for credit losses	0	0
Total bank loans – net	6,885,000,000	5,206,000,000
Pledged asset lines [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	4,000,000	4,000,000
Pledged asset lines [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	6,000,000	0
Pledged asset lines [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	0	0
Other [Member]
Financing Receivable, Past Due [Line Items]
Current	176,000,000	201,000,000
Past due and other nonaccrual loans	1,000,000	2,000,000
Total loans	177,000,000	203,000,000
Allowance for credit losses	3,000,000	3,000,000	$ 4,000,000	$ 3,000,000	$ 2,000,000	$ 2,000,000
Total bank loans – net	174,000,000	200,000,000
Other [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	0	0
Other [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	0	0
Other [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Past Due [Line Items]
Past due and other nonaccrual loans	1,000,000	2,000,000
First Mortgage and HELOC [Member]
Financing Receivable, Past Due [Line Items]
Unamortized premiums and discounts and direct origination costs	$ 75,000,000	$ 74,000,000
First Mortgage and HELOC [Member] | Loans, Geographic Area [Member] | California [Member]
Financing Receivable, Past Due [Line Items]
Concentration risk percentage	45.00%	45.00%