Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2016	Dec. 31, 2023
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Condensed Balance Sheet Statements, Captions [Line Items]
Change in Deferred Gain on Retroactive Reinsurance	$ 185			$ 209
Reinsurance Contract [Axis]: Asbestos and Environmental | Asbestos and Environmental
Condensed Balance Sheet Statements, Captions [Line Items]
Reinsurance, Amount Retained, Per Policy			$ 1,700
Change in Deferred Gain on Retroactive Reinsurance	788
Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Increase (Decrease) in Deferred Revenue	(24)	$ 0
Segment Reconciling Items | Asbestos and Environmental
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	0
Segment Reconciling Items | Catastrophe
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	0
Accident and Health Insurance Product Line [Member] | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	(67)	(61)
Accident and Health Insurance Product Line [Member] | Segment Reconciling Items | COVID-19 related Claims
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	20	20
Change in Workers Compensation Discount Including Accretion [Member] | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	12	11
General Liability [Member] | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	17	12
Marine | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	7	1
Package Business [Member] | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	(5)
Property Insurance [Member] | Commercial Lines | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	(3)	5
Property Insurance [Member] | Personal Lines | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	(1)
Professional Liability Insurance [Member] | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	(5)	0
Surety Product Line [Member] | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	0
Assumed reinsurance | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	9	2
Automobiles [Member] | Commercial Lines | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	0
Automobiles [Member] | Personal Lines | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	0
Uncollectible reinsurance | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	0	8
Insurance, Other [Member] | Segment Reconciling Items
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	(2)	28
Property, Liability and Casualty Insurance Product Line
Condensed Balance Sheet Statements, Captions [Line Items]
Prior accident year development [1]	(56)	0
Prior accident year development before change in deferred gain	(32)	0
Property, Liability and Casualty Insurance Product Line | Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Condensed Balance Sheet Statements, Captions [Line Items]
Increase (Decrease) in Deferred Revenue	$ (24)	$ 0