Bank Loans and Related Allowance for Loan Losses (Composition of Bank Loans and Delinquency Analysis by Loan Segment) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 16,213		$ 15,376
Past due and other nonaccrual loans	45		53
Total loans	16,258		15,429
Allowance for loan losses	26	$ 26	26	$ 26	$ 31	$ 31
Total bank loans - net	16,232		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	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	26		26
Residential real estate mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,773		9,100
Past due and other nonaccrual loans	29		34
Total loans	9,802		9,134
Allowance for loan losses	16	17	17	15	20	20
Total bank loans - net	9,786		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	11		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	16		16
Home equity loans and lines of credit [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,027		2,336
Past due and other nonaccrual loans	15		14
Total loans	2,042		2,350
Allowance for loan losses	8	8	8	10	10	11
Total bank loans - net	2,034		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	4		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	10		10
Pledged asset lines [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,278		3,846
Past due and other nonaccrual loans	1		5
Total loans	4,279		3,851
Allowance for loan losses	0		0
Total bank loans - net	4,279		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	135		94
Past due and other nonaccrual loans	0		0
Total loans	135		94
Allowance for loan losses	2	$ 1	1	$ 1	$ 1	$ 0
Total bank loans - net	133		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