Bank Loans and Related Allowance for Credit Losses - Credit Quality Indicators of Bank Loan Portfolio (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Gross charge-offs
Total loans	$ 0	$ 0
Residential real estate
Gross charge-offs
Total loans	0	0
Residential real estate | First Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2024	631
2023	2,490		$ 2,694
2022	5,987		6,069
2021	11,481		11,630
2020	3,915		3,974
pre-2020	1,697		1,786
Total loans	26,201		26,153
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
pre-2020	0		0
Total loans	$ 0	0	$ 0
Percent of Loans on Nonaccrual Status
2024	0.01%
2023	0.01%		0.01%
2022	0.02%		0.02%
2021	0.01%		0.01%
2020	0.01%		0.01%
pre-2020	0.39%		0.19%
Total loans	0.04%		0.03%
Residential real estate | First Mortgages | Estimated Current Loan To Value Ratio 70% and Below
Financing Receivable, Credit Quality Indicator [Line Items]
2024	$ 457
2023	1,732		$ 1,853
2022	4,838		4,855
2021	11,239		11,341
2020	3,894		3,960
pre-2020	1,691		1,781
Total loans	23,851		23,790
Residential real estate | First Mortgages | Estimated Current Loan To Value Ratio Greater Than 70% Through 90%
Financing Receivable, Credit Quality Indicator [Line Items]
2024	174
2023	758		841
2022	1,124		1,185
2021	240		289
2020	20		14
pre-2020	6		5
Total loans	2,322		2,334
Residential real estate | First Mortgages | Estimated Current LTV Greater Than 90% Through 100%
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0
2023	0		0
2022	20		28
2021	2		0
2020	1		0
pre-2020	0		0
Total loans	23		28
Residential real estate | First Mortgages | Estimated Current LTV Greater Than 100%
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0
2023	0		0
2022	5		1
2021	0		0
2020	0		0
pre-2020	0		0
Total loans	5		1
Residential real estate | First Mortgages | Updated FICO Score Below 620
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0
2023	10		4
2022	15		15
2021	15		19
2020	16		8
pre-2020	17		17
Total loans	73		63
Residential real estate | First Mortgages | Updated FICO Score 620 Through 679
Financing Receivable, Credit Quality Indicator [Line Items]
2024	8
2023	37		46
2022	83		77
2021	106		87
2020	34		38
pre-2020	47		50
Total loans	315		298
Residential real estate | First Mortgages | Updated FICO Score 680 Through 739
Financing Receivable, Credit Quality Indicator [Line Items]
2024	61
2023	227		265
2022	533		575
2021	922		984
2020	299		316
pre-2020	160		169
Total loans	2,202		2,309
Residential real estate | First Mortgages | Updated FICO Score 740 and Above
Financing Receivable, Credit Quality Indicator [Line Items]
2024	562
2023	2,216		2,379
2022	5,356		5,402
2021	10,438		10,540
2020	3,566		3,612
pre-2020	1,473		1,550
Total loans	23,611		23,483
Residential real estate | First Mortgages | Financial Asset Originated
Financing Receivable, Credit Quality Indicator [Line Items]
2024	631
2023	2,490		2,694
2022	5,987		6,069
2021	11,481		11,630
2020	3,915		3,974
pre-2020	1,697		1,786
Total loans	26,201		26,153
Residential real estate | First Mortgages | Financial Asset Originated | Origination Loan to Value Ratio 70% and Below
Financing Receivable, Credit Quality Indicator [Line Items]
2024	453
2023	1,673		1,818
2022	4,434		4,492
2021	9,954		10,078
2020	3,257		3,306
pre-2020	1,314		1,382
Total loans	21,085		21,076
Residential real estate | First Mortgages | Financial Asset Originated | Origination Loan to Value Ratio Greater Than 70% Through 90%
Financing Receivable, Credit Quality Indicator [Line Items]
2024	178
2023	817		876
2022	1,553		1,577
2021	1,527		1,552
2020	658		668
pre-2020	381		402
Total loans	5,114		5,075
Residential real estate | First Mortgages | Financial Asset Originated | Origination Loan to Value Ratio Greater Than 90% Through 100%
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
pre-2020	2		2
Total loans	2		2
Residential real estate | First Mortgages | Financial Asset Originated | Origination FICO Score Below 620
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0
2023	0		0
2022	3		2
2021	0		1
2020	1		1
pre-2020	1		1
Total loans	5		5
Residential real estate | First Mortgages | Financial Asset Originated | Origination FICO Score 620 Through 679
Financing Receivable, Credit Quality Indicator [Line Items]
2024	5
2023	5		4
2022	26		27
2021	30		30
2020	19		20
pre-2020	15		15
Total loans	100		96
Residential real estate | First Mortgages | Financial Asset Originated | Origination FICO Score 680 Through 739
Financing Receivable, Credit Quality Indicator [Line Items]
2024	67
2023	280		299
2022	773		782
2021	1,147		1,160
2020	390		395
pre-2020	278		294
Total loans	2,935		2,930
Residential real estate | First Mortgages | Financial Asset Originated | Origination FICO Score 740 and Above
Financing Receivable, Credit Quality Indicator [Line Items]
2024	559
2023	2,205		2,391
2022	5,185		5,258
2021	10,304		10,439
2020	3,505		3,558
pre-2020	1,403		1,476
Total loans	23,161		23,122
Residential real estate | HELOCs
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	299		311
HELOCs converted to term loans	156		168
Total loans	455		479
Gross charge-offs
Revolving HELOCs amortized cost basis	0		0
HELOCs converted to term loans	0		$ 0
Total loans	$ 0	$ 0
Percent of Loans on Nonaccrual Status
Revolving HELOCs amortized cost basis	0.13%		0.07%
HELOCs converted to term loans	2.36%		2.40%
Total loans	0.88%		0.84%
Residential real estate | HELOCs | Estimated Current Loan To Value Ratio 70% and Below
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	$ 296		$ 308
HELOCs converted to term loans	155		167
Total loans	451		475
Residential real estate | HELOCs | Estimated Current Loan To Value Ratio Greater Than 70% Through 90%
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	3		3
HELOCs converted to term loans	1		1
Total loans	4		4
Residential real estate | HELOCs | Estimated Current LTV Greater Than 90% Through 100%
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
Residential real estate | HELOCs | Estimated Current LTV Greater Than 100%
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
Residential real estate | HELOCs | Updated FICO Score Below 620
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	2		2
HELOCs converted to term loans	6		5
Total loans	8		7
Residential real estate | HELOCs | Updated FICO Score 620 Through 679
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	7		6
HELOCs converted to term loans	9		10
Total loans	16		16
Residential real estate | HELOCs | Updated FICO Score 680 Through 739
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	45		48
HELOCs converted to term loans	24		26
Total loans	69		74
Residential real estate | HELOCs | Updated FICO Score 740 and Above
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	245		255
HELOCs converted to term loans	117		127
Total loans	362		382
Residential real estate | HELOCs | Financial Asset Originated
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	299		311
HELOCs converted to term loans	156		168
Total loans	455		479
Residential real estate | HELOCs | Financial Asset Originated | Origination Loan to Value Ratio 70% and Below
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	271		279
HELOCs converted to term loans	109		117
Total loans	380		396
Residential real estate | HELOCs | Financial Asset Originated | Origination Loan to Value Ratio Greater Than 70% Through 90%
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	28		32
HELOCs converted to term loans	46		50
Total loans	74		82
Residential real estate | HELOCs | Financial Asset Originated | Origination Loan to Value Ratio Greater Than 90% Through 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	0		0
HELOCs converted to term loans	1		1
Total loans	1		1
Residential real estate | HELOCs | Financial Asset Originated | Origination FICO Score Below 620
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
Residential real estate | HELOCs | Financial Asset Originated | Origination FICO Score 620 Through 679
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	0		0
HELOCs converted to term loans	2		1
Total loans	2		1
Residential real estate | HELOCs | Financial Asset Originated | Origination FICO Score 680 Through 739
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	49		50
HELOCs converted to term loans	35		38
Total loans	84		88
Residential real estate | HELOCs | Financial Asset Originated | Origination FICO Score 740 and Above
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving HELOCs amortized cost basis	250		261
HELOCs converted to term loans	119		129
Total loans	$ 369		$ 390