Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 16,525	$ 15,201	$ 17,471
Other comprehensive income (loss) before reclassifications, after tax	289	826	(2,358)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	398	135	139
Other comprehensive income (loss)	687	961	(2,219)
Amounts attributable to noncontrolling interests	(57)	(77)	219
Purchase of CNA shares/ Other	0	(61)
Balance at end of period	17,937	16,525	15,201
Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(1,704)
Net Unrealized Gains (Losses) on Investments with an Allowance for Credit Losses
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(12)	(7)	(2)
Other comprehensive income (loss) before reclassifications, after tax	(34)	(24)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	33	19	(5)
Other comprehensive income (loss)	(1)	(5)	(5)
Amounts attributable to noncontrolling interests
Purchase of CNA shares/ Other
Balance at end of period	(13)	(12)	(7)
Other comprehensive income (loss) before reclassifications, tax	9	6	0
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax	(9)	(5)	1
Net Unrealized Gains (Losses) on Investments with an Allowance for Credit Losses | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period
Cumulative effect adjustments from changes in accounting standards, tax				$ 0
Net Unrealized Gains (Losses) on Investments with an Allowance for Credit Losses | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(2)
Net Unrealized Gains (Losses) on Other Investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(1,483)	(2,469)	930
Other comprehensive income (loss) before reclassifications, after tax	(309)	1,072	(6,223)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	47	53	126
Other comprehensive income (loss)	(262)	1,125	(6,097)
Amounts attributable to noncontrolling interests	22	(93)	619
Purchase of CNA shares/ Other	3	(46)
Balance at end of period	(1,720)	(1,483)	(2,469)
Other comprehensive income (loss) before reclassifications, tax	81	(290)	1,643
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax	(13)	(14)	(21)
Net Unrealized Gains (Losses) on Other Investments | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			2,079
Cumulative effect adjustments from changes in accounting standards, tax				(617)
Net Unrealized Gains (Losses) on Other Investments | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			3,009
Cumulative impact of changes in discount rates used to measure long duration contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(329)	(36)	0
Other comprehensive income (loss) before reclassifications, after tax	712	(318)	3,959
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax
Other comprehensive income (loss)	712	(318)	3,959
Amounts attributable to noncontrolling interests	(59)	26	(410)
Purchase of CNA shares/ Other		(1)
Balance at end of period	324	(329)	(36)
Other comprehensive income (loss) before reclassifications, tax	(189)	85	(1,052)
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax	0	0	0
Cumulative impact of changes in discount rates used to measure long duration contracts | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(3,585)
Cumulative effect adjustments from changes in accounting standards, tax				1,063
Cumulative impact of changes in discount rates used to measure long duration contracts | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(3,585)
Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	9	14	(6)
Other comprehensive income (loss) before reclassifications, after tax		(5)	20
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax
Other comprehensive income (loss)	0	(5)	20
Amounts attributable to noncontrolling interests
Purchase of CNA shares/ Other
Balance at end of period	9	9	14
Other comprehensive income (loss) before reclassifications, tax	(1)	2	(7)
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax	0	0	0
Unrealized Gains (Losses) on Cash Flow Hedges | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period
Cumulative effect adjustments from changes in accounting standards, tax				0
Unrealized Gains (Losses) on Cash Flow Hedges | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(6)
Pension and Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(533)	(622)	(636)
Other comprehensive income (loss) before reclassifications, after tax	22	41	(3)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	318	63	18
Other comprehensive income (loss)	340	104	15
Amounts attributable to noncontrolling interests	(28)	(5)	(1)
Purchase of CNA shares/ Other	(3)	(10)
Balance at end of period	(224)	(533)	(622)
Other comprehensive income (loss) before reclassifications, tax	(6)	(10)	1
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax	(86)	(18)	(5)
Pension and Postretirement Benefits | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period
Cumulative effect adjustments from changes in accounting standards, tax				0
Pension and Postretirement Benefits | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(636)
Foreign Currency Translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(149)	(200)	(100)
Other comprehensive income (loss) before reclassifications, after tax	(102)	60	(111)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax
Other comprehensive income (loss)	(102)	60	(111)
Amounts attributable to noncontrolling interests	8	(5)	11
Purchase of CNA shares/ Other		(4)
Balance at end of period	(243)	(149)	(200)
Other comprehensive income (loss) before reclassifications, tax	1	0	0
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax	0	0	0
Foreign Currency Translation | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period
Cumulative effect adjustments from changes in accounting standards, tax				$ 0
Foreign Currency Translation | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(100)
Total Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,497)	(3,320)	186
Balance at end of period	$ (1,867)	$ (2,497)	(3,320)
Total Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(1,506)
Total Accumulated Other Comprehensive Income (Loss) | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			$ (1,320)