Segment Information - Summary of underwriting income or loss by segment (Details) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) segment	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment				3
Gross premiums written		$ 5,382	$ 4,845	$ 11,315	$ 9,625
Premiums ceded		(1,601)	(1,417)	(3,449)	(2,773)
Net premiums written		3,781	3,428	7,866	6,852
Change in unearned premiums		(216)	(463)	(879)	(1,004)
Net premiums earned		3,565	2,965	6,987	5,848
Other underwriting income		3	6	15	16
Losses and loss adjustment expenses		(1,827)	(1,491)	(3,555)	(2,962)
Acquisition expenses		(633)	(561)	(1,240)	(1,094)
Other operating expenses		(346)	(313)	(709)	(632)
Underwriting income (loss)		762	606	1,498	1,176
Net investment income		364	242	691	441
Net realized gains (losses)		122	(123)	189	(106)
Equity in net income (loss) of investment funds accounted for using the equity method		167	69	266	117
Other income (loss)		8	3	22	14
Corporate expenses	[1]	(23)	(20)	(69)	(49)
Transaction costs and other	[1]	(18)	(1)	(25)	(2)
Amortization of intangible assets		(27)	(24)	(48)	(47)
Interest expense		(35)	(33)	(69)	(65)
Net foreign exchange gains (losses)		1	(5)	32	(23)
Income (loss) before income taxes and income (loss) from operating affiliates		1,321	714	2,487	1,456
Income tax expense (benefit)		(97)	(67)	(198)	(131)
Income (loss) from operating affiliates		45	22	100	61
Net income (loss)		1,269	669	2,389	1,386
Net (income) loss attributable to noncontrolling interests		0	2	0	0
Net income (loss) available to Arch		1,269	671	2,389	1,386
Preferred dividends		(10)	(10)	(20)	(20)
Net income (loss) available to Arch common shareholders		$ 1,259	$ 661	$ 2,369	$ 1,366
Underwriting Ratios
Loss ratio		51.20%	50.30%	50.90%	50.60%
Acquisition expense ratio		17.80%	18.90%	17.70%	18.70%
Other operating expense ratio		9.70%	10.60%	10.10%	10.80%
Combined ratio		78.70%	79.80%	78.70%	80.10%
Goodwill and intangible assets		$ 725	$ 775	$ 725	$ 775	$ 731
Operating segments | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	2,102	1,955	4,228	3,934
Premiums ceded	[2]	(544)	(501)	(1,128)	(1,043)
Net premiums written		1,558	1,454	3,100	2,891
Change in unearned premiums		(80)	(126)	(171)	(306)
Net premiums earned		1,478	1,328	2,929	2,585
Other underwriting income		0	0	0	0
Losses and loss adjustment expenses		(848)	(761)	(1,702)	(1,464)
Acquisition expenses		(288)	(264)	(564)	(509)
Other operating expenses		(233)	(195)	(468)	(390)
Underwriting income (loss)		$ 109	$ 108	$ 195	$ 222
Underwriting Ratios
Loss ratio		57.30%	57.30%	58.10%	56.60%
Acquisition expense ratio		19.50%	19.90%	19.20%	19.70%
Other operating expense ratio		15.80%	14.70%	16.00%	15.10%
Combined ratio		92.60%	91.90%	93.30%	91.40%
Goodwill and intangible assets		$ 255	$ 228	$ 255	$ 228
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	2,941	2,544	6,408	5,004
Premiums ceded	[2]	(994)	(835)	(2,195)	(1,569)
Net premiums written		1,947	1,709	4,213	3,435
Change in unearned premiums		(167)	(366)	(767)	(762)
Net premiums earned		1,780	1,343	3,446	2,673
Other underwriting income		1	3	3	7
Losses and loss adjustment expenses		(1,006)	(743)	(1,889)	(1,509)
Acquisition expenses		(345)	(290)	(676)	(571)
Other operating expenses		(64)	(68)	(139)	(142)
Underwriting income (loss)		$ 366	$ 245	$ 745	$ 458
Underwriting Ratios
Loss ratio		56.50%	55.30%	54.80%	56.50%
Acquisition expense ratio		19.40%	21.60%	19.60%	21.30%
Other operating expense ratio		3.60%	5.00%	4.00%	5.30%
Combined ratio		79.50%	81.90%	78.40%	83.10%
Goodwill and intangible assets		$ 114	$ 146	$ 114	$ 146
Operating segments | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	[2]	340	347	681	690
Premiums ceded	[2]	(64)	(82)	(128)	(164)
Net premiums written		276	265	553	526
Change in unearned premiums		31	29	59	64
Net premiums earned		307	294	612	590
Other underwriting income		2	3	12	9
Losses and loss adjustment expenses		27	13	36	11
Acquisition expenses		0	(7)	0	(14)
Other operating expenses		(49)	(50)	(102)	(100)
Underwriting income (loss)		$ 287	$ 253	$ 558	$ 496
Underwriting Ratios
Loss ratio		(8.60%)	(4.50%)	(5.80%)	(1.90%)
Acquisition expense ratio		0.10%	2.40%	0.10%	2.40%
Other operating expense ratio		15.90%	17.10%	16.70%	17.00%
Combined ratio		7.40%	15.00%	11.00%	17.50%
Goodwill and intangible assets		$ 356	$ 401	$ 356	$ 401

[1] Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘transaction costs and other.’

[2] 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.