Bank Loans and Related Allowance for Credit Losses (Credit Quality Indicators of Bank Loan Portfolio) (Details) $ in Millions	Jun. 30, 2020 USD ($) credit_rating	Dec. 31, 2019 USD ($) credit_rating
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 20,901	$ 18,230
First Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4,619
2019	3,580	4,002
2018	869	1,284
2017	1,476	1,911
2016	1,686	2,149
Pre-2016	1,763	2,358
Total loans	$ 13,993	$ 11,704
Percent of Loans on Nonaccrual Status
2020	0.07%
2019	0.00%	0.04%
2018	0.03%	0.04%
2017	0.03%	0.04%
2016	0.08%	0.08%
Pre-2016	0.45%	0.25%
Percent of Loans on Nonaccrual Status	0.09%	0.09%
First Mortgage [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 4,619
2019	3,580	$ 4,002
2018	869	1,284
2017	1,476	1,911
2016	1,686	2,149
Pre-2016	1,763	2,358
Total loans	13,993	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	3,866
2019	2,797	3,104
2018	612	906
2017	1,106	1,427
2016	1,429	1,812
Pre-2016	1,240	1,679
Total loans	11,050	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	753
2019	783	898
2018	257	378
2017	370	484
2016	257	337
Pre-2016	520	676
Total loans	2,940	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	3,912
2019	2,975	3,125
2018	744	1,018
2017	1,430	1,790
2016	1,671	2,119
Pre-2016	1,744	2,330
Total loans	12,476	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	707
2019	605	877
2018	125	265
2017	46	121
2016	15	30
Pre-2016	17	27
Total loans	1,515	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	1	1
Total loans	1	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	12
2019	14	12
2018	4	6
2017	11	14
2016	17	20
Pre-2016	18	25
Total loans	76	77
First Mortgage [Member] | Origination FICO Score 680 Through 739 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	491
2019	436	478
2018	153	220
2017	247	304
2016	235	290
Pre-2016	324	421
Total loans	1,886	1,713
First Mortgage [Member] | Origination FICO Score 740 and Above [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4,116
2019	3,130	3,512
2018	712	1,058
2017	1,218	1,593
2016	1,434	1,839
Pre-2016	1,419	1,909
Total loans	12,029	9,911
First Mortgage [Member] | Weighted Average Updated FICO Score Below 620 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4
2019	4	5
2018	2	4
2017	2	5
2016	4	3
Pre-2016	22	25
Total loans	38	42
First Mortgage [Member] | Wighted Average Updated FICO Score 620 Through 679 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	31
2019	54	45
2018	17	36
2017	33	32
2016	19	26
Pre-2016	54	68
Total loans	208	207
First Mortgage [Member] | Weighted Average Updated FICO Score 680 Through 739 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	430
2019	322	474
2018	104	153
2017	162	213
2016	142	199
Pre-2016	215	307
Total loans	1,375	1,346
First Mortgage [Member] | Weighted Average Updated FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4,154
2019	3,200	3,478
2018	746	1,091
2017	1,279	1,661
2016	1,521	1,921
Pre-2016	1,472	1,958
Total loans	12,372	10,109
HELOCs [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	535	611
HELOCs converted to term loans	446	506
Total loans	$ 981	$ 1,117
Percent of Loans on Nonaccrual Status
Revolving HELOCs amortized cost basis	0.21%	0.19%
HELOCs converted to term loans	1.58%	1.57%
Percent of Loans on Nonaccrual Status	0.71%	0.83%
HELOCs [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	$ 535	$ 611
HELOCs converted to term loans	446	506
Total loans	981	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	393	444
HELOCs converted to term loans	314	354
Total loans	707	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	142	167
HELOCs converted to term loans	128	147
Total loans	270	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	510	578
HELOCs converted to term loans	427	478
Total loans	937	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	25	33
HELOCs converted to term loans	16	23
Total loans	41	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	4	4
Total loans	5	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	100	114
HELOCs converted to term loans	94	105
Total loans	194	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	434	496
HELOCs converted to term loans	348	397
Total loans	782	893
HELOCs [Member] | Weighted Average Updated FICO Score Below 620 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	5	6
HELOCs converted to term loans	13	15
Total loans	18	21
HELOCs [Member] | Wighted Average Updated FICO Score 620 Through 679 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	14	18
HELOCs converted to term loans	20	22
Total loans	34	40
HELOCs [Member] | Weighted Average Updated FICO Score 680 Through 739 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	71	92
HELOCs converted to term loans	66	80
Total loans	137	172
HELOCs [Member] | Weighted Average Updated FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	445	495
HELOCs converted to term loans	347	389
Total loans	792	884
Pledged Asset Lines [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	5,734	5,206
Pledged Asset Lines [Member] | Weighted Average Loan to Value Ratio =70% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 5,734	$ 5,206
Percent of Loans on Nonaccrual Status
Percent of Loans on Nonaccrual Status	0.00%	0.00%
Weighted Average Updated FICO | credit_rating	770	766