Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net premiums earned		$ 9,678,077	$ 8,082,298	$ 6,991,935
Net premiums written		11,077,189	9,018,337	7,437,716
Operating segments | Insurance
Segment Reporting Information [Line Items]
Net premiums earned		4,559,335	3,626,468	2,871,420
Net premiums written		5,020,642	4,148,193	3,162,907
Operating segments | Insurance | United States
Segment Reporting Information [Line Items]
Net premiums written	[1]	3,340,038	2,813,039	2,158,415
Operating segments | Insurance | Europe
Segment Reporting Information [Line Items]
Net premiums written	[1]	1,405,719	1,125,192	856,572
Operating segments | Insurance | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written	[1]	274,885	209,962	147,920
Operating segments | Insurance | Professional lines
Segment Reporting Information [Line Items]
Net premiums earned	[1]	1,314,236	942,817	655,872
Operating segments | Insurance | Property, energy, marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	[1]	772,388	667,892	506,182
Operating segments | Insurance | Programs
Segment Reporting Information [Line Items]
Net premiums earned	[1]	589,860	506,867	432,854
Operating segments | Insurance | Travel, accident and health
Segment Reporting Information [Line Items]
Net premiums earned	[1]	491,847	255,590	190,944
Operating segments | Insurance | Construction and national accounts
Segment Reporting Information [Line Items]
Net premiums earned	[1]	432,020	416,107	398,999
Operating segments | Insurance | Excess and surplus casualty
Segment Reporting Information [Line Items]
Net premiums earned	[1]	393,353	318,027	270,620
Operating segments | Insurance | Warranty and lenders solutions
Segment Reporting Information [Line Items]
Net premiums earned	[1]	127,222	153,958	114,687
Operating segments | Insurance | Other
Segment Reporting Information [Line Items]
Net premiums earned	[1]	438,409	365,210	301,262
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned		3,959,381	2,840,443	2,162,229
Net premiums written		4,923,976	3,254,374	2,457,370
Operating segments | Reinsurance | United States
Segment Reporting Information [Line Items]
Net premiums written	[1]	1,246,507	828,504	687,622
Operating segments | Reinsurance | Bermuda
Segment Reporting Information [Line Items]
Net premiums written	[1]	2,561,771	1,557,294	1,001,990
Operating segments | Reinsurance | Europe and other
Segment Reporting Information [Line Items]
Net premiums written	[1]	1,115,698	868,576	767,758
Operating segments | Reinsurance | Other specialty
Segment Reporting Information [Line Items]
Net premiums earned	[1]	1,377,880	818,801	626,409
Operating segments | Reinsurance | Property excluding property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[1]	1,091,440	836,573	562,208
Operating segments | Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	[1]	854,543	666,754	549,056
Operating segments | Reinsurance | Property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[1]	366,991	280,738	237,736
Operating segments | Reinsurance | Marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	[1]	159,401	152,955	109,624
Operating segments | Reinsurance | Other
Segment Reporting Information [Line Items]
Net premiums earned	[1]	109,126	84,622	77,196
Operating segments | Mortgage
Segment Reporting Information [Line Items]
Net premiums earned		1,159,361	1,283,419	1,397,935
Net premiums written		1,132,571	1,261,068	1,279,850
Operating segments | Mortgage | United States
Segment Reporting Information [Line Items]
Net premiums earned		815,519	970,507	1,158,563
Net premiums written		780,256	914,477	1,021,950
Operating segments | Mortgage | Other geographic location
Segment Reporting Information [Line Items]
Net premiums earned		343,842	312,912	239,372
Net premiums written		352,315	346,591	257,900
Operating segments | Other
Segment Reporting Information [Line Items]
Net premiums earned		0	331,968	560,351
Net premiums written		0	354,702	537,589
Operating segments | Other | United States
Segment Reporting Information [Line Items]
Net premiums written	[1]	0	63,403	115,471
Operating segments | Other | Europe
Segment Reporting Information [Line Items]
Net premiums written	[1]	0	91,499	97,753
Operating segments | Other | Bermuda
Segment Reporting Information [Line Items]
Net premiums written	[1]	0	199,800	324,365
Operating segments | Other | Other specialty
Segment Reporting Information [Line Items]
Net premiums earned	[1]	0	118,356	186,717
Operating segments | Other | Property excluding property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[1]	0	6,578	1,130
Operating segments | Other | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	[1]	0	138,551	245,272
Operating segments | Other | Property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[1]	0	15,235	23,037
Operating segments | Other | Marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	[1]	0	190	429
Operating segments | Other | Other
Segment Reporting Information [Line Items]
Net premiums earned	[1]	$ 0	$ 53,058	$ 103,766

[1] Segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.