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, 2023 USD ($) segment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Segment Reporting Information [Line Items]
Gross premiums written		$ 18,403	$ 15,327	$ 12,752
Premiums ceded		(4,935)	(4,249)	(3,735)
Net premiums written		13,468	11,078	9,017
Change in unearned premiums		(1,028)	(1,399)	(935)
Net premiums earned		12,440	9,679	8,082
Other underwriting income		31	13	21
Losses and loss adjustment expenses		(6,246)	(5,028)	(4,585)
Acquisition expenses, net		(2,312)	(1,740)	(1,303)
Other operating expenses		(1,301)	(1,128)	(999)
Underwriting income (loss)		2,612	1,796	1,216
Net investment income		1,023	496	389
Net realized gains (losses)		(165)	(663)	380
Equity in net income (loss) of investment funds accounted for using the equity method		278	115	366
Other income (loss)		27	(27)	10
Corporate expenses		(96)	(95)	(76)
Transaction costs and other		(6)	0	(2)
Amortization of intangible assets		(95)	(106)	(82)
Interest expense		(133)	(131)	(139)
Net foreign exchange gains (losses)		(60)	102	41
Income before income taxes and income (loss) from operating affiliates		3,385	1,487	2,103
Income tax (expense) benefit		873	(80)	(128)
Income (loss) from operating affiliates		184	75	264
Net income (loss)		4,442	1,482	2,239
Amounts attributable to redeemable noncontrolling interests		1	(6)	(4)
Amounts attributable to nonredeemable noncontrolling interests		0	0	(78)
Net income available to Arch		4,443	1,476	2,157
Preferred share dividends		(40)	(40)	(48)
Loss on redemption of preferred shares		0	0	(15)
Net income (loss) available to Arch common shareholders		$ 4,403	$ 1,436	$ 2,094
Underwriting Ratios
Loss ratio		50.20%	51.90%	56.70%
Acquisition expense ratio		18.60%	18.00%	16.10%
Other operating expense ratio		10.50%	11.70%	12.40%
Combined ratio		79.30%	81.60%	85.20%
Goodwill and intangible assets		$ 731	$ 804	$ 945
Total investable assets		34,589	28,065	27,442
Total assets		58,906	47,990	45,101
Total liabilities		$ 40,551	35,069	31,546
Operating segments | Underwriting Segments
Segment Reporting Information [Line Items]
Number of segments | segment		3
Gross premiums written	[1]	$ 18,403	15,327	12,463
Premiums ceded	[1]	(4,935)	(4,249)	(3,799)
Net premiums written		13,468	11,078	8,664
Change in unearned premiums		(1,028)	(1,399)	(915)
Net premiums earned		12,440	9,679	7,749
Other underwriting income		31	13	21
Losses and loss adjustment expenses		(6,246)	(5,028)	(4,327)
Acquisition expenses, net		(2,312)	(1,740)	(1,240)
Other operating expenses		(1,301)	(1,128)	(966)
Underwriting income (loss)		2,612	1,796	1,237
Net investment income		1,023	496	347
Net realized gains (losses)		(165)	(663)	299
Equity in net income (loss) of investment funds accounted for using the equity method		278	115	366
Other income (loss)		27	(27)	10
Corporate expenses		(96)	(95)	(76)
Transaction costs and other		(6)	0	(1)
Amortization of intangible assets		(95)	(106)	(81)
Interest expense		(133)	(131)	(131)
Net foreign exchange gains (losses)		(60)	102	42
Income before income taxes and income (loss) from operating affiliates		3,385	1,487	2,012
Income tax (expense) benefit		873	(80)	(128)
Income (loss) from operating affiliates		184	75	264
Net income (loss)		4,442	1,482	2,148
Amounts attributable to redeemable noncontrolling interests		1	(6)	(2)
Amounts attributable to nonredeemable noncontrolling interests				0
Net income available to Arch		4,443	1,476	2,148
Preferred share dividends		(40)	(40)	(48)
Loss on redemption of preferred shares				(15)
Net income (loss) available to Arch common shareholders		$ 4,403	$ 1,436	$ 2,085
Underwriting Ratios
Loss ratio		50.20%	51.90%	55.80%
Acquisition expense ratio		18.60%	18.00%	16.00%
Other operating expense ratio		10.50%	11.70%	12.50%
Combined ratio		79.30%	81.60%	84.30%
Goodwill and intangible assets		$ 731	$ 804	$ 945
Total investable assets		34,589	28,065	27,442
Total assets		58,906	47,990	45,101
Total liabilities		40,551	35,069	31,546
Operating segments | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	[1]	7,911	6,931	5,868
Premiums ceded	[1],[2]	(2,049)	(1,910)	(1,719)
Net premiums written		5,862	5,021	4,149
Change in unearned premiums		(416)	(461)	(524)
Net premiums earned		5,446	4,560	3,625
Other underwriting income		0	0	0
Losses and loss adjustment expenses		(3,122)	(2,784)	(2,345)
Acquisition expenses, net		(1,055)	(887)	(606)
Other operating expenses		(819)	(665)	(559)
Underwriting income (loss)		$ 450	$ 224	$ 115
Underwriting Ratios
Loss ratio		57.30%	61.00%	64.60%
Acquisition expense ratio		19.40%	19.40%	16.70%
Other operating expense ratio		15.00%	14.60%	15.40%
Combined ratio		91.70%	95.00%	96.70%
Goodwill and intangible assets		$ 224	$ 229	$ 256
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	[1]	9,113	6,948	5,094
Premiums ceded	[1],[2]	(2,559)	(2,024)	(1,840)
Net premiums written		6,554	4,924	3,254
Change in unearned premiums		(718)	(965)	(413)
Net premiums earned		5,836	3,959	2,841
Other underwriting income		17	5	3
Losses and loss adjustment expenses		(3,227)	(2,568)	(1,925)
Acquisition expenses, net		(1,240)	(813)	(537)
Other operating expenses		(288)	(268)	(214)
Underwriting income (loss)		$ 1,098	$ 315	$ 168
Underwriting Ratios
Loss ratio		55.30%	64.90%	67.80%
Acquisition expense ratio		21.20%	20.50%	18.90%
Other operating expense ratio		4.90%	6.80%	7.50%
Combined ratio		81.40%	92.20%	94.20%
Goodwill and intangible assets		$ 130	$ 145	$ 184
Operating segments | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	[1]	1,387	1,455	1,508
Premiums ceded	[1],[2]	(335)	(322)	(247)
Net premiums written		1,052	1,133	1,261
Change in unearned premiums		106	27	22
Net premiums earned		1,158	1,160	1,283
Other underwriting income		14	8	18
Losses and loss adjustment expenses		103	324	(57)
Acquisition expenses, net		(17)	(40)	(97)
Other operating expenses		(194)	(195)	(193)
Underwriting income (loss)		$ 1,064	$ 1,257	$ 954
Underwriting Ratios
Loss ratio		(8.90%)	(28.00%)	4.40%
Acquisition expense ratio		1.40%	3.50%	7.60%
Other operating expense ratio		16.80%	16.80%	15.10%
Combined ratio		9.30%	(7.70%)	27.10%
Goodwill and intangible assets		$ 377	$ 430	$ 505
Operating segments | Other
Segment Reporting Information [Line Items]
Number of segments | segment		2
Gross premiums written	[1]	$ 0	0	458
Premiums ceded	[1]	0	0	(105)	[2]
Net premiums written		0	0	353
Change in unearned premiums		0	0	(20)
Net premiums earned		0	0	333
Other underwriting income		0	0	0
Losses and loss adjustment expenses		0	0	(258)
Acquisition expenses, net		0	0	(63)
Other operating expenses		0	0	(33)
Underwriting income (loss)		0	0	(21)
Net investment income		0	0	42
Net realized gains (losses)		0	0	81
Equity in net income (loss) of investment funds accounted for using the equity method		0	0	0
Other income (loss)		0	0	0
Corporate expenses		0	0	0
Transaction costs and other		0	0	(1)
Amortization of intangible assets		0	0	(1)
Interest expense		0	0	(8)
Net foreign exchange gains (losses)		0	0	(1)
Income before income taxes and income (loss) from operating affiliates		0	0	91
Income tax (expense) benefit		0	0	0
Income (loss) from operating affiliates		0	0	0
Net income (loss)		0	0	91
Amounts attributable to redeemable noncontrolling interests		0	0	(2)
Amounts attributable to nonredeemable noncontrolling interests				(78)
Net income available to Arch		0	0	9
Preferred share dividends		0	0	0
Loss on redemption of preferred shares				0
Net income (loss) available to Arch common shareholders		$ 0	$ 0	$ 9
Underwriting Ratios
Loss ratio		0.00%	0.00%	78.00%
Acquisition expense ratio		0.00%	0.00%	18.90%
Other operating expense ratio		0.00%	0.00%	9.90%
Combined ratio		0.00%	0.00%	106.80%
Goodwill and intangible assets		$ 0	$ 0	$ 0
Total investable assets		0	0	0
Total assets		0	0	0
Total liabilities		$ 0	$ 0	$ 0

[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.