Bank Loans and Related Allowance for Loan Losses (Credit Quality Indicators of Bank Loan Portfolio) (Details) $ in Millions	Sep. 30, 2019 USD ($) credit_rating	Dec. 31, 2018 USD ($) credit_rating
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 16,912	$ 16,630
First Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 10,640	$ 10,384
Weighted Average Updated FICO | credit_rating	776	775
Percent of Loans on Nonaccrual Status	0.05%	0.07%
First Mortgage [Member] | Origination FICO Score Below 620 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 4	$ 5
First Mortgage [Member] | Origination FICO Score 620 Through 679 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	76	83
First Mortgage [Member] | Origination FICO Score 680 Through 739 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	1,624	1,626
First Mortgage [Member] | Origination FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	8,936	8,670
First Mortgage [Member] | Year of origination Pre 2015 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	1,815	2,387
First Mortgage [Member] | Year of origination 2015 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	889	1,050
First Mortgage [Member] | Year of origination 2016 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	2,382	2,606
First Mortgage [Member] | Year of origination 2017 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	2,137	2,366
First Mortgage [Member] | Year of origination 2018 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	1,611	1,975
First Mortgage [Member] | Year of origination 2019 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	1,806
First Mortgage [Member] | Estimated Current LTV 70% and Below [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 9,574	$ 9,396
Weighted Average Updated FICO | credit_rating	776	776
Percent of Loans on Nonaccrual Status	0.03%	0.04%
First Mortgage [Member] | Estimated Current LTV Greater Than 70% through 90% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 1,061	$ 985
Weighted Average Updated FICO | credit_rating	770	769
Percent of Loans on Nonaccrual Status	0.28%	0.41%
First Mortgage [Member] | Estimated Current LTV Greater Than 90% through 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 4	$ 2
Weighted Average Updated FICO | credit_rating	756	717
Percent of Loans on Nonaccrual Status	0.00%	0.00%
First Mortgage [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 1	$ 1
Weighted Average Updated FICO | credit_rating	768	753
Percent of Loans on Nonaccrual Status	0.00%	0.00%
First Mortgage [Member] | Origination Loan to Value Ratio 70% and Below [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 7,980	$ 7,815
First Mortgage [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	2,656	2,564
First Mortgage [Member] | Origination Loan to Value Ratio Greater Than 90% Through 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	4	5
HELOCs [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 1,220	$ 1,505
Weighted Average Updated FICO | credit_rating	768	769
Percent of Loans on Nonaccrual Status	0.29%	0.17%
HELOCs [Member] | Origination FICO Score Below 620 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 0	$ 0
HELOCs [Member] | Origination FICO Score 620 Through 679 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	6	8
HELOCs [Member] | Origination FICO Score 680 Through 739 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	236	282
HELOCs [Member] | Origination FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	978	1,215
HELOCs [Member] | Year of origination Pre 2015 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	872	1,140
HELOCs [Member] | Year of origination 2015 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	85	106
HELOCs [Member] | Year of origination 2016 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	82	95
HELOCs [Member] | Year of origination 2017 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	90	99
HELOCs [Member] | Year of origination 2018 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	67	65
HELOCs [Member] | Year of origination 2019 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	24
HELOCs [Member] | Estimated Current LTV 70% and Below [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 1,157	$ 1,416
Weighted Average Updated FICO | credit_rating	769	770
Percent of Loans on Nonaccrual Status	0.25%	0.13%
HELOCs [Member] | Estimated Current LTV Greater Than 70% through 90% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 58	$ 80
Weighted Average Updated FICO | credit_rating	751	752
Percent of Loans on Nonaccrual Status	0.83%	0.60%
HELOCs [Member] | Estimated Current LTV Greater Than 90% through 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 3	$ 6
Weighted Average Updated FICO | credit_rating	716	729
Percent of Loans on Nonaccrual Status	5.06%	3.36%
HELOCs [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 2	$ 3
Weighted Average Updated FICO | credit_rating	659	702
Percent of Loans on Nonaccrual Status	0.00%	0.00%
HELOCs [Member] | Origination Loan to Value Ratio 70% and Below [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 869	$ 1,064
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	346	434
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 90% Through 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	5	7
Pledged asset lines [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	4,878	4,561
Pledged asset lines [Member] | Weighted Average Loan to Value Ratio =70% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Balance	$ 4,878	$ 4,561
Weighted Average Updated FICO | credit_rating	767	766
Percent of Loans on Nonaccrual Status	0.00%	0.00%