Bank Loans and Related Allowance for Credit Losses (Credit Quality Indicators of Bank Loan Portfolio) (Details) $ in Millions	Sep. 30, 2020 USD ($) credit_rating	Dec. 31, 2019 USD ($) credit_rating
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 22,316	$ 18,230
First Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	6,091
2019	3,256	4,002
2018	718	1,284
2017	1,283	1,911
2016	1,466	2,149
Pre-2016	1,526	2,358
Total loans	$ 14,340	$ 11,704
Percent of Loans on Nonaccrual Status
2020	0.09%
2019	0.05%	0.04%
2018	0.01%	0.04%
2017	0.15%	0.04%
2016	0.17%	0.08%
Pre-2016	0.99%	0.25%
Percent of Loans on Nonaccrual Status	0.18%	0.09%
First Mortgage [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 6,091
2019	3,256	$ 4,002
2018	718	1,284
2017	1,283	1,911
2016	1,466	2,149
Pre-2016	1,526	2,358
Total loans	14,340	11,704
First Mortgage [Member] | Origination Loan to Value Ratio 70% and Below [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,064
2019	2,552	3,104
2018	506	906
2017	967	1,427
2016	1,243	1,812
Pre-2016	1,081	1,679
Total loans	11,413	8,928
First Mortgage [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,027
2019	704	898
2018	212	378
2017	316	484
2016	223	337
Pre-2016	442	676
Total loans	2,924	2,773
First Mortgage [Member] | Origination Loan to Value Ratio Greater Than 90% Through 100% [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0	0
2018	0	0
2017	0	0
2016	0	0
Pre-2016	3	3
Total loans	3	3
First Mortgage [Member] | Estimated Current Loan To Value Ratio 70% and Below [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,198
2019	2,802	3,125
2018	650	1,018
2017	1,251	1,790
2016	1,457	2,119
Pre-2016	1,512	2,330
Total loans	12,870	10,382
First Mortgage [Member] | Estimated Current Loan To Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	893
2019	454	877
2018	68	265
2017	32	121
2016	9	30
Pre-2016	13	27
Total loans	1,469	1,320
First Mortgage [Member] | Estimated Current LTV Greater Than 90% Through 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0	0
2018	0	1
2017	0	0
2016	0	0
Pre-2016	0	1
Total loans	0	2
First Mortgage [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0	0
2018	0	0
2017	0	0
2016	0	0
Pre-2016	1	0
Total loans	1	0
First Mortgage [Member] | Origination FICO Score Below 620 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0	0
2018	0	0
2017	0	0
2016	0	0
Pre-2016	2	3
Total loans	2	3
First Mortgage [Member] | Origination FICO Score 620 Through 679 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	22
2019	14	12
2018	4	6
2017	11	14
2016	17	20
Pre-2016	17	25
Total loans	85	77
First Mortgage [Member] | Origination FICO Score 680 Through 739 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	626
2019	405	478
2018	132	220
2017	214	304
2016	200	290
Pre-2016	287	421
Total loans	1,864	1,713
First Mortgage [Member] | Origination FICO Score 740 and Above [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,443
2019	2,837	3,512
2018	582	1,058
2017	1,058	1,593
2016	1,249	1,839
Pre-2016	1,220	1,909
Total loans	12,389	9,911
First Mortgage [Member] | Weighted Average Updated FICO Score Below 620 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	6
2019	4	5
2018	2	4
2017	3	5
2016	1	3
Pre-2016	18	25
Total loans	34	42
First Mortgage [Member] | Wighted Average Updated FICO Score 620 Through 679 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	47
2019	37	45
2018	18	36
2017	24	32
2016	20	26
Pre-2016	38	68
Total loans	184	207
First Mortgage [Member] | Weighted Average Updated FICO Score 680 Through 739 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	514
2019	300	474
2018	92	153
2017	148	213
2016	129	199
Pre-2016	196	307
Total loans	1,379	1,346
First Mortgage [Member] | Weighted Average Updated FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,524
2019	2,915	3,478
2018	606	1,091
2017	1,108	1,661
2016	1,316	1,921
Pre-2016	1,274	1,958
Total loans	12,743	10,109
HELOCs [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	498	611
HELOCs converted to term loans	416	506
Total loans	$ 914	$ 1,117
Percent of Loans on Nonaccrual Status
Revolving HELOCs amortized cost basis	0.63%	0.19%
HELOCs converted to term loans	2.74%	1.57%
Percent of Loans on Nonaccrual Status	1.64%	0.83%
HELOCs [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	$ 498	$ 611
HELOCs converted to term loans	416	506
Total loans	914	1,117
HELOCs [Member] | Origination Loan to Value Ratio 70% and Below [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	368	444
HELOCs converted to term loans	294	354
Total loans	662	798
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 70% Through 90% [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	130	167
HELOCs converted to term loans	118	147
Total loans	248	314
HELOCs [Member] | Origination Loan to Value Ratio Greater Than 90% Through 100% [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	0	0
HELOCs converted to term loans	4	5
Total loans	4	5
HELOCs [Member] | Estimated Current Loan To Value Ratio 70% and Below [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	478	578
HELOCs converted to term loans	400	478
Total loans	878	1,056
HELOCs [Member] | Estimated Current Loan To Value Ratio Greater Than 70% Through 90% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	20	33
HELOCs converted to term loans	13	23
Total loans	33	56
HELOCs [Member] | Estimated Current LTV Greater Than 90% Through 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	0	0
HELOCs converted to term loans	2	3
Total loans	2	3
HELOCs [Member] | Estimated Current LTV Greater Than 100% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	0	0
HELOCs converted to term loans	1	2
Total loans	1	2
HELOCs [Member] | Origination FICO Score Below 620 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	0	0
HELOCs converted to term loans	0	0
Total loans	0	0
HELOCs [Member] | Origination FICO Score 620 Through 679 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	1	1
HELOCs converted to term loans	3	4
Total loans	4	5
HELOCs [Member] | Origination FICO Score 680 Through 739 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	91	114
HELOCs converted to term loans	88	105
Total loans	179	219
HELOCs [Member] | Origination FICO Score 740 and Above [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	406	496
HELOCs converted to term loans	325	397
Total loans	731	893
HELOCs [Member] | Weighted Average Updated FICO Score Below 620 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	3	6
HELOCs converted to term loans	11	15
Total loans	14	21
HELOCs [Member] | Wighted Average Updated FICO Score 620 Through 679 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	15	18
HELOCs converted to term loans	22	22
Total loans	37	40
HELOCs [Member] | Weighted Average Updated FICO Score 680 Through 739 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	65	92
HELOCs converted to term loans	61	80
Total loans	126	172
HELOCs [Member] | Weighted Average Updated FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	415	495
HELOCs converted to term loans	322	389
Total loans	737	884
Pledged Asset Lines [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	6,885	5,206
Pledged Asset Lines [Member] | Weighted Average Loan to Value Ratio =70% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 6,885	$ 5,206
Percent of Loans on Nonaccrual Status
Percent of Loans on Nonaccrual Status	0.00%	0.00%
Weighted Average Updated FICO | credit_rating	772	766