Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period				$ 17,471	$ 15,201	$ 17,471	$ 16,835
Other comprehensive income (loss) before reclassifications, after tax					826	(2,358)	241
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax					135	139	(26)
Other comprehensive income (loss)	$ 400	$ (839)	$ (778)	(1,002)	961	(2,219)	215
Amounts attributable to noncontrolling interests					(77)	219	(19)
Purchase of CNA shares					(61)
Balance at end of period	15,201				16,525	15,201	17,471
Cumulative Effect, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period							(2,346)
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				(2)	(7)	(2)	0
Other comprehensive income (loss) before reclassifications, after tax					(24)		(7)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax					19	(5)	5
Other comprehensive income (loss)					(5)	(5)	(2)
Amounts attributable to noncontrolling interests
Purchase of CNA shares
Balance at end of period	(7)				(12)	(7)	(2)
Other comprehensive income (loss) before reclassifications, tax					6	0	2
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax					(5)	1	(1)
Net Unrealized Gains (Losses) on Investments with an Allowance for Credit Losses | Cumulative Effect, 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							0
Net Unrealized Gains (Losses) on Other Investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period				3,009	(2,469)	3,009	1,563
Other comprehensive income (loss) before reclassifications, after tax					1,072	(6,223)	(906)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax					53	126	(81)
Other comprehensive income (loss)					1,125	(6,097)	(987)
Amounts attributable to noncontrolling interests					(93)	619	102
Purchase of CNA shares					(46)
Balance at end of period	(2,469)				(1,483)	(2,469)	3,009
Other comprehensive income (loss) before reclassifications, tax					(290)	1,643	242
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax					(14)	(21)	21
Net Unrealized Gains (Losses) on Other Investments | Cumulative Effect, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period							2,331
Cumulative effect adjustments from changes in accounting standards, tax								(691)
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							3,894
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				(3,585)	(36)	(3,585)	0
Other comprehensive income (loss) before reclassifications, after tax					(318)	3,959	941
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax
Other comprehensive income (loss)					(318)	3,959	941
Amounts attributable to noncontrolling interests					26	(410)	(98)
Purchase of CNA shares					(1)
Balance at end of period	(36)				(329)	(36)	(3,585)
Other comprehensive income (loss) before reclassifications, tax					85	(1,052)	(250)
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, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period							(4,428)
Cumulative effect adjustments from changes in accounting standards, tax								1,313
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							(4,428)
Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period				(6)	14	(6)	(23)
Other comprehensive income (loss) before reclassifications, after tax					(5)	20	13
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax							4
Other comprehensive income (loss)					(5)	20	17
Amounts attributable to noncontrolling interests
Purchase of CNA shares
Balance at end of period	14				9	14	(6)
Other comprehensive income (loss) before reclassifications, tax					2	(7)	(2)
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax					0	0	(1)
Unrealized Gains (Losses) on Cash Flow Hedges | Cumulative Effect, 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							(23)
Pension and Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period				(636)	(622)	(636)	(877)
Other comprehensive income (loss) before reclassifications, after tax					41	(3)	220
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax					63	18	46
Other comprehensive income (loss)					104	15	266
Amounts attributable to noncontrolling interests					(5)	(1)	(25)
Purchase of CNA shares					(10)
Balance at end of period	(622)				(533)	(622)	(636)
Other comprehensive income (loss) before reclassifications, tax					(10)	1	(59)
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax					(18)	(5)	(12)
Pension and Postretirement Benefits | Cumulative Effect, 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							(877)
Foreign Currency Translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period				(100)	(200)	(100)	(82)
Other comprehensive income (loss) before reclassifications, after tax					60	(111)	(20)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax
Other comprehensive income (loss)					60	(111)	(20)
Amounts attributable to noncontrolling interests					(5)	11	2
Purchase of CNA shares					(4)
Balance at end of period	(200)				(149)	(200)	(100)
Other comprehensive income (loss) before reclassifications, tax					0	0	0
Reclassification of (gains) losses from accumulated other comprehensive income (loss), tax					0	0	0
Foreign Currency Translation | Cumulative Effect, 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							(82)
Total Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period				$ (1,320)	(3,320)	(1,320)	581
Balance at end of period	$ (3,320)				$ (2,497)	$ (3,320)	(1,320)
Total Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period							(2,097)
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							$ (1,516)