Bank Loans and Related Allowance for Loan Losses (Composition of Bank Loans and Delinquency Analysis by Loan Segment) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 16,362,000,000	$ 16,448,000,000
Past due and other nonaccrual loans	54,000,000	56,000,000
Total loans	16,416,000,000	16,504,000,000
Allowance for loan losses	27,000,000	26,000,000	$ 26,000,000	$ 26,000,000
Total bank loans - net	16,389,000,000	16,478,000,000
Loans 90 days past due and still accruing	0	0
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	20,000,000	18,000,000
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	5,000,000	9,000,000
More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	29,000,000	29,000,000
First Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,041,000,000	9,983,000,000
Past due and other nonaccrual loans	37,000,000	33,000,000
Total loans	10,078,000,000	10,016,000,000
Allowance for loan losses	17,000,000	16,000,000	17,000,000	17,000,000
Total bank loans - net	10,061,000,000	10,000,000,000
First Mortgage [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	15,000,000	14,000,000
First Mortgage [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	3,000,000	2,000,000
First Mortgage [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	19,000,000	17,000,000
HELOCs [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,781,000,000	1,928,000,000
Past due and other nonaccrual loans	15,000,000	15,000,000
Total loans	1,796,000,000	1,943,000,000
Allowance for loan losses	7,000,000	8,000,000	8,000,000	8,000,000
Total bank loans - net	1,789,000,000	1,935,000,000
HELOCs [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	4,000,000	0
HELOCs [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	1,000,000	3,000,000
HELOCs [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	10,000,000	12,000,000
Pledged asset lines [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,360,000,000	4,361,000,000
Past due and other nonaccrual loans	2,000,000	8,000,000
Total loans	4,362,000,000	4,369,000,000
Allowance for loan losses	0	0
Total bank loans - net	4,362,000,000	4,369,000,000
Pledged asset lines [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	1,000,000	4,000,000
Pledged asset lines [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	1,000,000	4,000,000
Pledged asset lines [Member] | More Than 90 Days Past Due and Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	0	0
Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	180,000,000	176,000,000
Past due and other nonaccrual loans	0	0
Total loans	180,000,000	176,000,000
Allowance for loan losses	3,000,000	2,000,000	$ 1,000,000	$ 1,000,000
Total bank loans - net	177,000,000	174,000,000
Other [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	0	0
Other [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, 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, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	0	0
First Mortgage and HELOC [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unamortized premiums and discounts and direct origination costs	$ 75,000,000	$ 77,000,000
First Mortgage and HELOC [Member] | Loans, Geographic Area [Member] | California [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Concentration risk percentage	48.00%	48.00%