Segment Information - Analysis of underwriting income by segment and reconciliation to net income available to common shareholders (Details) $ in Millions		12 Months Ended
		Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Gross premiums written		$ 21,511	$ 18,403	$ 15,327
Premiums ceded		(5,779)	(4,935)	(4,249)
Net premiums written		15,732	13,468	11,078
Change in unearned premiums		(632)	(1,028)	(1,399)
Net premiums earned		15,100	12,440	9,679
Other underwriting income		26	31	13
Losses and loss adjustment expenses		(8,342)	(6,246)	(5,028)
Acquisition expenses, net		(2,651)	(2,312)	(1,740)
Other operating expenses		(1,472)	(1,301)	(1,128)
Underwriting income (loss)		2,661	2,612	1,796
Net investment income		1,495	1,023	496
Net realized gains (losses)		197	(165)	(663)
Equity in net income (loss) of investment funds accounted for using the equity method		580	278	115
Other income (loss)		42	27	(27)
Corporate expenses		(119)	(96)	(95)
Transaction costs and other		(81)	(6)	0
Amortization of intangible assets		(235)	(95)	(106)
Interest expense		(141)	(133)	(131)
Net foreign exchange gains (losses)		75	(60)	102
Income before income taxes and income (loss) from operating affiliates		4,474	3,385	1,487
Income tax (expense) benefit		(362)	873	(80)
Income (loss) from operating affiliates		200	184	75
Net income (loss)		4,312	4,442	1,482
Amounts attributable to redeemable noncontrolling interests			1	(6)
Net income available to Arch		4,312	4,443	1,476
Preferred share dividends		(40)	(40)	(40)
Net income (loss) available to Arch common shareholders		$ 4,272	$ 4,403	$ 1,436
Underwriting Ratios
Loss ratio		55.20%	50.20%	51.90%
Acquisition expense ratio		17.60%	18.60%	18.00%
Other operating expense ratio		9.70%	10.50%	11.70%
Combined ratio		82.50%	79.30%	81.60%
Goodwill and intangible assets		$ 1,351	$ 731	$ 804
Total investable assets		41,388	34,589	28,065
Total assets		70,906	58,906	47,990
Total liabilities		$ 50,086	40,551	35,069
Operating segments
Segment Reporting Information [Line Items]
Number of segments | segment		3
Operating segments | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	[1]	$ 9,053	7,911	6,931
Premiums ceded	[1],[2]	(2,179)	(2,049)	(1,910)
Net premiums written		6,874	5,862	5,021
Change in unearned premiums		(247)	(416)	(461)
Net premiums earned		6,627	5,446	4,560
Other underwriting income		0	0	0
Losses and loss adjustment expenses		(4,070)	(3,122)	(2,784)
Acquisition expenses, net		(1,217)	(1,055)	(887)
Other operating expenses	[3]	(995)	(819)	(665)
Underwriting income (loss)		$ 345	$ 450	$ 224
Underwriting Ratios
Loss ratio		61.40%	57.30%	61.00%
Acquisition expense ratio		18.40%	19.40%	19.40%
Other operating expense ratio		15.00%	15.00%	14.60%
Combined ratio		94.80%	91.70%	95.00%
Goodwill and intangible assets		$ 916	$ 224	$ 229
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	[1]	11,112	9,113	6,948
Premiums ceded	[1],[2]	(3,366)	(2,559)	(2,024)
Net premiums written		7,746	6,554	4,924
Change in unearned premiums		(504)	(718)	(965)
Net premiums earned		7,242	5,836	3,959
Other underwriting income		9	17	5
Losses and loss adjustment expenses		(4,327)	(3,227)	(2,568)
Acquisition expenses, net		(1,432)	(1,240)	(813)
Other operating expenses	[3]	(270)	(288)	(268)
Underwriting income (loss)		$ 1,222	$ 1,098	$ 315
Underwriting Ratios
Loss ratio		59.70%	55.30%	64.90%
Acquisition expense ratio		19.80%	21.20%	20.50%
Other operating expense ratio		3.70%	4.90%	6.80%
Combined ratio		83.20%	81.40%	92.20%
Goodwill and intangible assets		$ 102	$ 130	$ 145
Operating segments | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	[1]	1,351	1,387	1,455
Premiums ceded	[1],[2]	(239)	(335)	(322)
Net premiums written		1,112	1,052	1,133
Change in unearned premiums		119	106	27
Net premiums earned		1,231	1,158	1,160
Other underwriting income		17	14	8
Losses and loss adjustment expenses		55	103	324
Acquisition expenses, net		(2)	(17)	(40)
Other operating expenses	[3]	(207)	(194)	(195)
Underwriting income (loss)		$ 1,094	$ 1,064	$ 1,257
Underwriting Ratios
Loss ratio		(4.40%)	(8.90%)	(28.00%)
Acquisition expense ratio		0.20%	1.40%	3.50%
Other operating expense ratio		16.80%	16.80%	16.80%
Combined ratio		12.60%	9.30%	(7.70%)
Goodwill and intangible assets		$ 333	$ 377	$ 430

[1] Certain assumed and ceded amounts related to intersegment transactions are included in individual segment results. Accordingly, the sum of such transactions for each segment does not agree to the total due to eliminations.

[2] Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.

[3] Other operating expenses primarily include expenses that are related to compensation and employee benefits, information technology and professional fees.