Bank Loans and Related Allowance for Loan Losses (Composition of Bank Loans and Delinquency Analysis by Loan Segment) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 15,799		$ 15,376
Past due and other nonaccrual loans	44		53
Total loans	15,843		15,429
Allowance for loan losses	26	$ 26	26	$ 31	$ 33	$ 31
Total bank loans — net	15,817		15,403
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	16		21
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	2		6
>90 Days Past Due And Other Nonaccrual Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due and other nonaccrual loans	26		26
Residential real estate mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,549		9,100
Past due and other nonaccrual loans	32		34
Total loans	9,581		9,134
Allowance for loan losses	17	17	17	20	21	20
Total bank loans — net	9,564		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	14		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	1		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	2,137		2,336
Past due and other nonaccrual loans	11		14
Total loans	2,148		2,350
Allowance for loan losses	8	8	8	10	11	11
Total bank loans — net	2,140		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	1		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	1		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	9		10
Pledged Asset Lines [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,000		3,846
Past due and other nonaccrual loans	1		5
Total loans	4,001		3,851
Allowance for loan losses	0		0
Total bank loans — net	4,001		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	1		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	0		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	113		94
Past due and other nonaccrual loans	0		0
Total loans	113		94
Allowance for loan losses	1	$ 1	1	$ 1	$ 1	$ 0
Total bank loans — net	112		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