Bank Loans and Related Allowance for Credit Losses (Credit Quality Indicators of Bank Loan Portfolio) (Details) $ in Millions	Mar. 31, 2020 USD ($) credit_rating	Dec. 31, 2019 USD ($) credit_rating
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 19,550	$ 18,230
First Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,952
2019	3,992	4,002
2018	1,085	1,284
2017	1,739	1,911
2016	1,979	2,149
Pre-2016	2,051	2,358
Total loans	$ 12,798	$ 11,704
Percent of Loans on Nonaccrual Status
2020	0.03%
2019	0.03%	0.04%
2018	0.03%	0.04%
2017	0.05%	0.04%
2016	0.08%	0.08%
Pre-2016	0.34%	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	$ 1,952
2019	3,992	$ 4,002
2018	1,085	1,284
2017	1,739	1,911
2016	1,979	2,149
Pre-2016	2,051	2,358
Total loans	12,798	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	1,578
2019	3,120	3,104
2018	771	906
2017	1,290	1,427
2016	1,669	1,812
Pre-2016	1,453	1,679
Total loans	9,881	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	374
2019	872	898
2018	314	378
2017	449	484
2016	310	337
Pre-2016	594	676
Total loans	2,913	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	4	3
Total loans	4	3
First Mortgage [Member] | Estimated Current Loan To Value Ratio 70% and Below [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,578
2019	3,168	3,125
2018	884	1,018
2017	1,646	1,790
2016	1,956	2,119
Pre-2016	2,026	2,330
Total loans	11,258	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	374
2019	824	877
2018	199	265
2017	93	121
2016	23	30
Pre-2016	22	27
Total loans	1,535	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	2	1
2017	0	0
2016	0	0
Pre-2016	2	1
Total loans	4	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	3	3
Total loans	3	3
First Mortgage [Member] | Origination FICO Score 620 Through 679 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	11
2019	14	12
2018	6	6
2017	14	14
2016	19	20
Pre-2016	20	25
Total loans	84	77
First Mortgage [Member] | Origination FICO Score 680 Through 739 [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	222
2019	480	478
2018	193	220
2017	278	304
2016	271	290
Pre-2016	369	421
Total loans	1,813	1,713
First Mortgage [Member] | Origination FICO Score 740 and Above [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,719
2019	3,498	3,512
2018	886	1,058
2017	1,447	1,593
2016	1,689	1,839
Pre-2016	1,659	1,909
Total loans	10,898	9,911
First Mortgage [Member] | Weighted Average Updated FICO Score Below 620 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4
2019	5	5
2018	3	4
2017	5	5
2016	4	3
Pre-2016	24	25
Total loans	45	42
First Mortgage [Member] | Wighted Average Updated FICO Score 620 Through 679 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	14
2019	49	45
2018	23	36
2017	23	32
2016	21	26
Pre-2016	54	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	192
2019	385	474
2018	135	153
2017	184	213
2016	177	199
Pre-2016	264	307
Total loans	1,337	1,346
First Mortgage [Member] | Weighted Average Updated FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,742
2019	3,553	3,478
2018	924	1,091
2017	1,527	1,661
2016	1,777	1,921
Pre-2016	1,709	1,958
Total loans	11,232	10,109
HELOCs [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	592	611
HELOCs converted to term loans	475	506
Total loans	$ 1,067	$ 1,117
Percent of Loans on Nonaccrual Status
Revolving HELOCs amortized cost basis	0.14%	0.19%
HELOCs converted to term loans	1.69%	1.57%
Percent of Loans on Nonaccrual Status	0.84%	0.83%
HELOCs [Member] | Financial Asset Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	$ 592	$ 611
HELOCs converted to term loans	475	506
Total loans	1,067	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	432	444
HELOCs converted to term loans	331	354
Total loans	763	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	160	167
HELOCs converted to term loans	139	147
Total loans	299	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	5	5
Total loans	5	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	558	578
HELOCs converted to term loans	450	478
Total loans	1,008	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	34	33
HELOCs converted to term loans	20	23
Total loans	54	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	3	3
Total loans	3	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	2	2
Total loans	2	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	2	1
HELOCs converted to term loans	4	4
Total loans	6	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	108	114
HELOCs converted to term loans	100	105
Total loans	208	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	482	496
HELOCs converted to term loans	371	397
Total loans	853	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	14	15
Total loans	19	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	21	22
Total loans	35	40
HELOCs [Member] | Weighted Average Updated FICO Score 680 Through 739 [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	84	92
HELOCs converted to term loans	75	80
Total loans	159	172
HELOCs [Member] | Weighted Average Updated FICO Score 740 and Above [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	489	495
HELOCs converted to term loans	365	389
Total loans	854	884
Pledged Asset Lines [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	5,467	5,206
Pledged Asset Lines [Member] | Weighted Average Loan to Value Ratio =70% [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 5,467	$ 5,206
Percent of Loans on Nonaccrual Status
Percent of Loans on Nonaccrual Status	0.00%	0.00%
Weighted Average Updated FICO | credit_rating	769	766