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, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net premiums earned		$ 8,082,298	$ 6,991,935	$ 5,786,498
Net premiums written		9,018,337	7,437,716	6,039,067
Operating segments | Insurance
Segment Reporting Information [Line Items]
Net premiums earned		3,626,468	2,871,420	2,397,080
Net premiums written		4,148,193	3,162,907	2,641,726
Operating segments | Insurance | United States
Segment Reporting Information [Line Items]
Net premiums written	[1]	2,813,039	2,158,415	1,983,476
Operating segments | Insurance | Europe
Segment Reporting Information [Line Items]
Net premiums written	[1]	1,125,192	856,572	559,214
Operating segments | Insurance | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written	[1]	209,962	147,920	99,036
Operating segments | Insurance | Professional lines
Segment Reporting Information [Line Items]
Net premiums earned	[1],[2]	942,817	655,872	499,224
Operating segments | Insurance | Property, energy, marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	[1]	702,693	517,247	298,966
Operating segments | Insurance | Programs
Segment Reporting Information [Line Items]
Net premiums earned	[1]	506,867	432,854	414,103
Operating segments | Insurance | Construction and national accounts
Segment Reporting Information [Line Items]
Net premiums earned	[1]	381,306	387,934	325,687
Operating segments | Insurance | Excess and surplus casualty
Segment Reporting Information [Line Items]
Net premiums earned	[1],[3]	318,027	270,620	200,615
Operating segments | Insurance | Travel, accident and health
Segment Reporting Information [Line Items]
Net premiums earned	[1]	255,590	190,944	305,085
Operating segments | Insurance | Lenders products
Segment Reporting Information [Line Items]
Net premiums earned	[1]	153,958	114,687	66,079
Operating segments | Insurance | Other
Segment Reporting Information [Line Items]
Net premiums earned	[1],[4]	365,210	301,262	287,321
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned		2,840,443	2,162,229	1,466,389
Net premiums written		3,254,374	2,457,370	1,602,723
Operating segments | Reinsurance | United States
Segment Reporting Information [Line Items]
Net premiums written	[5]	828,504	687,622	529,943
Operating segments | Reinsurance | Bermuda
Segment Reporting Information [Line Items]
Net premiums written	[5]	1,557,294	1,001,990	578,618
Operating segments | Reinsurance | Europe and other
Segment Reporting Information [Line Items]
Net premiums written	[5]	868,576	767,758	494,162
Operating segments | Reinsurance | Property excluding property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[5],[6]	836,573	562,208	362,841
Operating segments | Reinsurance | Other specialty
Segment Reporting Information [Line Items]
Net premiums earned	[5],[6]	818,801	626,409	478,517
Operating segments | Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	[5],[7]	666,754	549,056	429,288
Operating segments | Reinsurance | Property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[5],[7]	280,738	237,736	90,934
Operating segments | Reinsurance | Marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	[5]	152,955	109,624	48,274
Operating segments | Reinsurance | Other
Segment Reporting Information [Line Items]
Net premiums earned	[5],[8]	84,622	77,196	56,535
Operating segments | Mortgage
Segment Reporting Information [Line Items]
Net premiums earned		1,283,419	1,397,935	1,366,340
Net premiums written		1,261,068	1,279,850	1,261,756
Operating segments | Mortgage | United States
Segment Reporting Information [Line Items]
Net premiums earned		970,507	1,158,563	1,134,849
Net premiums written		914,477	1,021,950	1,032,868
Operating segments | Mortgage | Other geographic location
Segment Reporting Information [Line Items]
Net premiums earned		312,912	239,372	231,491
Net premiums written		346,591	257,900	228,888
Operating segments | Other
Segment Reporting Information [Line Items]
Net premiums earned		331,968	560,351	556,689
Net premiums written		354,702	537,589	532,862
Operating segments | Other | United States
Segment Reporting Information [Line Items]
Net premiums written	[9]	63,403	115,471	127,176
Operating segments | Other | Europe
Segment Reporting Information [Line Items]
Net premiums written	[9]	91,499	97,753	52,065
Operating segments | Other | Bermuda
Segment Reporting Information [Line Items]
Net premiums written	[9]	199,800	324,365	353,621
Operating segments | Other | Property excluding property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[9]	6,578	1,130	3,503
Operating segments | Other | Other specialty
Segment Reporting Information [Line Items]
Net premiums earned	[9],[10]	118,356	186,717	185,547
Operating segments | Other | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	[9],[11]	138,551	245,272	246,894
Operating segments | Other | Property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	[9]	15,235	23,037	13,399
Operating segments | Other | Marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	[9]	190	429	0
Operating segments | Other | Other
Segment Reporting Information [Line Items]
Net premiums earned	[9],[12]	$ 53,058	$ 103,766	$ 107,346

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

[2] Includes professional liability, executive assurance and healthcare business.

[3] Includes casualty and contract binding business.

[4] Includes alternative markets, excess workers' compensation and surety business.

[5] Reinsurance segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.

[6] Includes proportional motor, surety, accident and health, workers’ compensation catastrophe, agriculture, trade credit and other.

[7] Includes executive assurance, professional liability, workers’ compensation, excess motor, healthcare and other.

[8] Includes life, casualty clash and other.

[9] Other segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.

[10] Includes proportional motor and other.

[11] Includes professional liability, excess motor, programs and other.

[12] Includes mortgage, US programs and other.