Bank Loans and Related Allowance for Loan Losses (Credit Quality Indicators of Bank Loan Portfolio) (Details) $ in Millions	Sep. 30, 2018 USD ($) credit_rating	Dec. 31, 2017 USD ($) credit_rating
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 16,590	$ 16,504
First Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 10,256	$ 10,016
Weighted Average Updated FICO | credit_rating	776	775
Percent of Loans on Nonaccrual Status	0.07%	0.14%
First Mortgage [Member] | Origination FICO Score Below 620 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 5	$ 6
First Mortgage [Member] | Origination FICO Score 620 Through 679 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	83	89
First Mortgage [Member] | Origination FICO Score 680 Through 739 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,595	1,569
First Mortgage [Member] | Origination FICO Score 740 and Above [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	8,573	8,352
First Mortgage [Member] | Year of origination Pre 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,144	2,804
First Mortgage [Member] | Year of origination 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	436	530
First Mortgage [Member] | Year of origination 2015 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,087	1,218
First Mortgage [Member] | Year of origination 2016 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,662	2,886
First Mortgage [Member] | Year of origination 2017 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,420	2,578
First Mortgage [Member] | Year of origination 2018 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,507
First Mortgage [Member] | Estimated Current LTV 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 9,383	$ 9,046
Weighted Average Updated FICO | credit_rating	777	775
Percent of Loans on Nonaccrual Status	0.05%	0.09%
First Mortgage [Member] | Estimated Current LTV Greater Than 70% through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 868	$ 961
Weighted Average Updated FICO | credit_rating	771	769
Percent of Loans on Nonaccrual Status	0.24%	0.46%
First Mortgage [Member] | Estimated Current LTV Greater Than 90% through 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 4	$ 5
Weighted Average Updated FICO | credit_rating	713	714
Percent of Loans on Nonaccrual Status	8.70%	10.49%
First Mortgage [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 1	$ 4
Weighted Average Updated FICO | credit_rating	742	713
Percent of Loans on Nonaccrual Status	0.00%	6.23%
First Mortgage [Member] | Origination Loan to Value Ratio 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 7,737	$ 7,569
First Mortgage [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,514	2,441
First Mortgage [Member] | Origination Loan to Value Ratio Greater Than 90% Through 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	5	6
HELOCs [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 1,602	$ 1,943
Weighted Average Updated FICO | credit_rating	770	770
Utilization Rate	31.00%	33.00%
Percent of Loans on Nonaccrual Status	0.24%	0.27%
HELOCs [Member] | Origination FICO Score Below 620 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 0	$ 1
HELOCs [Member] | Origination FICO Score 620 Through 679 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	8	10
HELOCs [Member] | Origination FICO Score 680 Through 739 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	305	365
HELOCs [Member] | Origination FICO Score 740 and Above [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,289	1,567
HELOCs [Member] | Year of origination Pre 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,156	1,496
HELOCs [Member] | Year of origination 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	93	116
HELOCs [Member] | Year of origination 2015 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	110	128
HELOCs [Member] | Year of origination 2016 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	96	111
HELOCs [Member] | Year of origination 2017 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	101	92
HELOCs [Member] | Year of origination 2018 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	46
HELOCs [Member] | Estimated Current LTV 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 1,509	$ 1,773
Weighted Average Updated FICO | credit_rating	771	772
Utilization Rate	31.00%	32.00%
Percent of Loans on Nonaccrual Status	0.18%	0.18%
HELOCs [Member] | Estimated Current LTV Greater Than 70% through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 84	$ 148
Weighted Average Updated FICO | credit_rating	752	755
Utilization Rate	47.00%	47.00%
Percent of Loans on Nonaccrual Status	0.90%	0.84%
HELOCs [Member] | Estimated Current LTV Greater Than 90% through 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 5	$ 14
Weighted Average Updated FICO | credit_rating	746	742
Utilization Rate	77.00%	64.00%
Percent of Loans on Nonaccrual Status	0.90%	2.85%
HELOCs [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 4	$ 8
Weighted Average Updated FICO | credit_rating	704	718
Utilization Rate	81.00%	72.00%
Percent of Loans on Nonaccrual Status	5.28%	4.91%
HELOCs [Member] | Origination Loan to Value Ratio 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 1,127	$ 1,360
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	468	574
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 90% Through 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	7	9
Pledged asset lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	4,556	4,369
Pledged asset lines [Member] | Weighted Average Loan to Value Ratio =70% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 4,556	$ 4,369
Weighted Average Updated FICO | credit_rating	766	765
Utilization Rate	36.00%	41.00%
Percent of Loans on Nonaccrual Status	0.00%	0.00%