Bank Loans and Related Allowance for Loan Losses (Composition of Bank Loans and Delinquency Analysis by Loan Segment) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 15,532	$ 15,376
Past due and other nonaccrual loans	42	53
Total loans	15,574	15,429
Allowance for loan losses	26	26	$ 33	$ 31
Total bank loans — net	15,548	15,403
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	10	21
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	3	6
>90 Days Past Due And Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	29	26
Residential real estate mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,371	9,100
Past due and other nonaccrual loans	27	34
Total loans	9,398	9,134
Allowance for loan losses	17	17	21	20
Total bank loans — net	9,381	9,117
Residential real estate mortgages [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	8	15
Residential real estate mortgages [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	2	3
Residential real estate mortgages [Member] | >90 Days Past Due And Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	17	16
Home equity loans and lines of credit [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,216	2,336
Past due and other nonaccrual loans	14	14
Total loans	2,230	2,350
Allowance for loan losses	8	8	11	11
Total bank loans — net	2,222	2,342
Home equity loans and lines of credit [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	2	2
Home equity loans and lines of credit [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	0	2
Home equity loans and lines of credit [Member] | >90 Days Past Due And Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	12	10
Pledged Asset Lines [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,844	3,846
Past due and other nonaccrual loans	1	5
Total loans	3,845	3,851
Allowance for loan losses	0	0
Total bank loans — net	3,845	3,851
Pledged Asset Lines [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	0	4
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	1
Pledged Asset Lines [Member] | >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	101	94
Past due and other nonaccrual loans	0	0
Total loans	101	94
Allowance for loan losses	1	1	$ 1	$ 0
Total bank loans — net	100	93
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] | >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