Segment Information (Tables)

9 Months Ended

Sep. 30, 2025

Segment Reporting [Abstract]

Analysis of underwriting income or loss by segment and reconciliation to net income available to common shareholders

The following tables summarize the Company’s underwriting income or loss by segment, together with a reconciliation of underwriting income or loss to net income available to Arch common shareholders:

	Three Months Ended
	September 30, 2025
	Insurance			Reinsurance			Mortgage			Total
Gross premiums written (1)	$	2,567		$	2,515		$	330		$	5,410
Premiums ceded (1)	(614)			(778)			(56)			(1,446)
Net premiums written	1,953			1,737			274			3,964
Change in unearned premiums	16			278			27			321
Net premiums earned	1,969			2,015			301			4,285
Other underwriting income (2)	9			38			3			50
Losses and loss adjustment expenses	(1,162)			(1,040)			2			(2,200)
Acquisition expenses	(386)			(398)			(2)			(786)
Other operating expenses (3)	(301)			(133)			(44)			(478)
Underwriting income (loss)	$	129		$	482		$	260		871
Net investment income										408
Net realized gains (losses)										210
Equity in net income of investments accounted for using the equity method										134
Other income (loss)										22
Corporate expenses (4)										(28)
Transaction costs and other (4)										(21)
Amortization of intangible assets										(49)
Interest expense										(37)
Net foreign exchange gains (losses)										(7)
Income (loss) before income taxes and income (loss) from operating affiliates										1,503
Income tax (expense) benefit										(215)
Income (loss) from operating affiliates										62
Net income (loss) available to Arch										1,350
Preferred dividends										(10)
Net income (loss) available to Arch common shareholders										$	1,340
Underwriting Ratios
Loss ratio	59.0		%	51.6		%	(0.5)		%	51.4		%
Acquisition expense ratio	19.6		%	19.8		%	0.7		%	18.4		%
Other operating expense ratio (5)	14.8		%	4.7		%	13.3		%	10.0		%
Combined ratio	93.4		%	76.1		%	13.5		%	79.8		%
Goodwill and intangible assets	$	833		$	100		$	335		$	1,268

(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)    ‘Other underwriting income’ includes revenue earned from underwriting-related activities covered under existing service contracts.

(3)    ‘Other operating expenses’ primarily include expenses that are related to compensation and employee benefits, information technology and professional fees.

(4)    Certain expenses have been excluded from ‘Corporate expenses’ and reflected in ‘Transaction costs and other.’

(5)    The ‘Other operating expense ratio’ for the 2025 period includes ‘Other underwriting income.’

	Three Months Ended
	September 30, 2024
	Insurance			Reinsurance			Mortgage			Total
Gross premiums written (1)	$	2,341		$	2,763		$	339		$	5,440
Premiums ceded (1)	(521)			(818)			(57)			(1,393)
Net premiums written	1,820			1,945			282			4,047
Change in unearned premiums	(55)			(53)			31			(77)
Net premiums earned	1,765			1,892			313			3,970
Other underwriting income	—			2			3			5
Losses and loss adjustment expenses	(1,087)			(1,317)			1			(2,403)
Acquisition expenses	(308)			(374)			1			(681)
Other operating expenses (2)	(250)			(54)			(49)			(353)
Underwriting income (loss)	$	120		$	149		$	269		538
Net investment income										399
Net realized gains (losses)										169
Equity in net income of investments accounted for using the equity method										171
Other income (loss)										8
Corporate expenses (3)										(19)
Transaction costs and other (3)										(30)
Amortization of intangible assets										(88)
Interest expense										(35)
Net foreign exchange gains (losses)										(63)
Income (loss) before income taxes and income (loss) from operating affiliates										1,050
Income tax (expense) benefit										(98)
Income (loss) from operating affiliates										36
Net income (loss) available to Arch										988
Preferred dividends										(10)
Net income (loss) available to Arch common shareholders										$	978
Underwriting Ratios
Loss ratio	61.6		%	69.6		%	(0.4)		%	60.5		%
Acquisition expense ratio	17.4		%	19.8		%	(0.4)		%	17.2		%
Other operating expense ratio	14.1		%	2.9		%	15.6		%	8.9		%
Combined ratio	93.1		%	92.3		%	14.8		%	86.6		%
Goodwill and intangible assets	$	1,025		$	113		$	348		$	1,486

(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)    ‘Other operating expenses’ primarily include expenses that are related to compensation and employee benefits, information technology and professional fees.

(3)    Certain expenses have been excluded from ‘Corporate expenses’ and reflected in ‘Transaction costs and other.’

	Nine Months Ended
	September 30, 2025
	Insurance			Reinsurance			Mortgage			Total
Gross premiums written (1)	$	7,893		$	9,205		$	979		$	18,069
Premiums ceded (1)	(1,971)			(3,093)			(186)			(5,242)
Net premiums written	5,922			6,112			793			12,827
Change in unearned premiums	(124)			18			89			(17)
Net premiums earned	5,798			6,130			882			12,810
Other underwriting income (2)	25			123			17			165
Losses and loss adjustment expenses	(3,568)			(3,524)			2			(7,090)
Acquisition expenses	(1,116)			(1,251)			(7)			(2,374)
Other operating expenses (3)	(883)			(378)			(144)			(1,405)
Underwriting income (loss)	$	256		$	1,100		$	750		2,106
Net investment income										1,191
Net realized gains (losses)										442
Equity in net income of investments accounted for using the equity method										349
Other income (loss)										38
Corporate expenses (4)										(107)
Transaction costs and other (4)										(49)
Amortization of intangible assets										(146)
Interest expense										(110)
Net foreign exchange gains (losses)										(122)
Income (loss) before income taxes and income (loss) from operating affiliates										3,592
Income tax (expense) benefit										(550)
Income (loss) from operating affiliates										119
Net income (loss) available to Arch										3,161
Preferred dividends										(30)
Net income (loss) available to Arch common shareholders										$	3,131
Underwriting Ratios
Loss ratio	61.5		%	57.5		%	(0.2)		%	55.4		%
Acquisition expense ratio	19.3		%	20.4		%	0.8		%	18.5		%
Other operating expense ratio (5)	14.8		%	4.2		%	14.3		%	9.7		%
Combined ratio	95.6		%	82.1		%	14.9		%	83.6		%

(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)    ‘Other underwriting income’ includes revenue earned from underwriting-related activities covered under existing service contracts.

(3)    ‘Other operating expenses’ primarily include expenses that are related to compensation and employee benefits, information technology and professional fees.

(4)    Certain expenses have been excluded from ‘Corporate expenses’ and reflected in ‘Transaction costs and other.’

(5)    The ‘Other operating expense ratio’ for the 2025 period includes ‘Other underwriting income.’

	Nine Months Ended
	September 30, 2024
	Insurance			Reinsurance			Mortgage			Total
Gross premiums written (1)	$	6,569		$	9,171		$	1,020		$	16,755
Premiums ceded (1)	(1,649)			(3,013)			(185)			(4,842)
Net premiums written	4,920			6,158			835			11,913
Change in unearned premiums	(226)			(820)			90			(956)
Net premiums earned	4,694			5,338			925			10,957
Other underwriting income	—			5			15			20
Losses and loss adjustment expenses	(2,789)			(3,206)			37			(5,958)
Acquisition expenses	(872)			(1,050)			1			(1,921)
Other operating expenses (2)	(718)			(193)			(151)			(1,062)
Underwriting income (loss)	$	315		$	894		$	827		2,036
Net investment income										1,090
Net realized gains (losses)										358
Equity in net income of investments accounted for using the equity method										437
Other income (loss)										30
Corporate expenses (3)										(88)
Transaction costs and other (3)										(55)
Amortization of intangible assets										(136)
Interest expense										(104)
Net foreign exchange gains (losses)										(31)
Income (loss) before income taxes and income (loss) from operating affiliates										3,537
Income tax (expense) benefit										(296)
Income (loss) from operating affiliates										136
Net income (loss) available to Arch										3,377
Preferred dividends										(30)
Net income (loss) available to Arch common shareholders										$	3,347
Underwriting Ratios
Loss ratio	59.4		%	60.1		%	(4.0)		%	54.4		%
Acquisition expense ratio	18.6		%	19.7		%	(0.1)		%	17.5		%
Other operating expense ratio	15.3		%	3.6		%	16.3		%	9.7		%
Combined ratio	93.3		%	83.4		%	12.2		%	81.6		%

(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)    ‘Other operating expenses’ primarily include expenses that are related to compensation and employee benefits, information technology and professional fees.

(3)    Certain expenses have been excluded from ‘Corporate expenses’ and reflected in ‘Transaction costs and other.’