Bank Loans and Related Allowance for Loan Losses (Credit Quality Indicators of Bank Loan Portfolio) (Details) $ in Millions	Jun. 30, 2018 USD ($) credit_rating	Dec. 31, 2017 USD ($) credit_rating
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 16,595	$ 16,504
First Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 10,158	$ 10,016
Weighted Average Updated FICO | credit_rating	776	775
Percent of Loans on Nonaccrual Status	0.10%	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	84	89
First Mortgage [Member] | Origination FICO Score 680 Through 739 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,574	1,569
First Mortgage [Member] | Origination FICO Score 740 and Above [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	8,495	8,352
First Mortgage [Member] | Year of origination Pre 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,313	2,804
First Mortgage [Member] | Year of origination 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	461	530
First Mortgage [Member] | Year of origination 2015 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,121	1,218
First Mortgage [Member] | Year of origination 2016 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,734	2,886
First Mortgage [Member] | Year of origination 2017 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,484	2,578
First Mortgage [Member] | Year of origination 2018 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,045
First Mortgage [Member] | Estimated Current LTV 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 9,287	$ 9,046
Weighted Average Updated FICO | credit_rating	776	775
Percent of Loans on Nonaccrual Status	0.06%	0.09%
First Mortgage [Member] | Estimated Current LTV Greater Than 70% through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 865	$ 961
Weighted Average Updated FICO | credit_rating	770	769
Percent of Loans on Nonaccrual Status	0.42%	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	696	714
Percent of Loans on Nonaccrual Status	8.72%	10.49%
First Mortgage [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 2	$ 4
Weighted Average Updated FICO | credit_rating	729	713
Percent of Loans on Nonaccrual Status	10.14%	6.23%
First Mortgage [Member] | Origination Loan to Value Ratio 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 7,677	$ 7,569
First Mortgage [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	2,476	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	$ 1,698	$ 1,943
Weighted Average Updated FICO | credit_rating	771	770
Utilization Rate	31.00%	33.00%
Percent of Loans on Nonaccrual Status	0.19%	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	9	10
HELOCs [Member] | Origination FICO Score 680 Through 739 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	321	365
HELOCs [Member] | Origination FICO Score 740 and Above [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,368	1,567
HELOCs [Member] | Year of origination Pre 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	1,252	1,496
HELOCs [Member] | Year of origination 2014 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	99	116
HELOCs [Member] | Year of origination 2015 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	113	128
HELOCs [Member] | Year of origination 2016 [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	101	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	32
HELOCs [Member] | Estimated Current LTV 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 1,580	$ 1,773
Weighted Average Updated FICO | credit_rating	772	772
Utilization Rate	31.00%	32.00%
Percent of Loans on Nonaccrual Status	0.11%	0.18%
HELOCs [Member] | Estimated Current LTV Greater Than 70% through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 105	$ 148
Weighted Average Updated FICO | credit_rating	754	755
Utilization Rate	48.00%	47.00%
Percent of Loans on Nonaccrual Status	1.04%	0.84%
HELOCs [Member] | Estimated Current LTV Greater Than 90% through 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 8	$ 14
Weighted Average Updated FICO | credit_rating	741	742
Utilization Rate	72.00%	64.00%
Percent of Loans on Nonaccrual Status	2.39%	2.85%
HELOCs [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 5	$ 8
Weighted Average Updated FICO | credit_rating	714	718
Utilization Rate	76.00%	72.00%
Percent of Loans on Nonaccrual Status	2.03%	4.91%
HELOCs [Member] | Origination Loan to Value Ratio 70% and Below [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 1,194	$ 1,360
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	496	574
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 90% Through 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	8	9
Pledged asset lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	4,570	4,369
Pledged asset lines [Member] | Weighted Average Loan to Value Ratio =70% [Member]
Financing Receivable, Recorded Investment [Line Items]
Balance	$ 4,570	$ 4,369
Weighted Average Updated FICO | credit_rating	766	765
Utilization Rate	38.00%	41.00%
Percent of Loans on Nonaccrual Status	0.00%	0.00%