Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	$ 14,351,402	$ 13,507,131
Short term investments	4,669,272	5,298,385
Equity investments	625,058	546,016
Other investments	2,494,954	1,993,059
Derivative assets	44,421	17,889
Derivative liabilities	(7,581)	(16,954)
Other assets	3,499	6,100
Other liabilities	(5,331)	(10,827)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	14,351,402	13,507,131
Short term investments	4,669,272	5,298,385
Equity investments	625,058	546,016
Other investments	2,494,954	1,993,059
Assumed and ceded (re)insurance contracts	(1,832)	(4,727)
Derivative assets	44,400	17,889
Derivative liabilities	(7,560)	(16,954)
Other assets	35,008
Other liabilities		(3,792)
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	22,175,694	21,340,799
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1, 2 and 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	1,408,248	1,267,257
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	7,180,129	6,247,779
Short term investments	0	0
Equity investments	625,058	546,016
Other investments	0	0
Assumed and ceded (re)insurance contracts	0	0
Derivative assets	387	1,067
Derivative liabilities	(2,008)	(1,598)
Other liabilities	(1,621)	(531)
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	7,803,566	6,793,264
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	7,171,273	7,259,352
Short term investments	4,669,272	5,298,385
Equity investments	0	0
Other investments	1,241,468	1,104,034
Assumed and ceded (re)insurance contracts	0	0
Derivative assets	44,013	16,822
Derivative liabilities	(5,552)	(15,356)
Other assets	38,461	1,466
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	13,120,474	13,663,237
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Short term investments	0	0
Equity investments	0	0
Other investments	166,780	163,223
Assumed and ceded (re)insurance contracts	(1,832)	(4,727)
Derivative assets	0	0
Derivative liabilities	0	0
Other liabilities	(1,832)	(4,727)
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	164,948	158,496
Fair Value, Measurements, Recurring | U.S. treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	7,180,129	6,247,779
Fair Value, Measurements, Recurring | U.S. treasuries | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	7,180,129	6,247,779
Fair Value, Measurements, Recurring | U.S. treasuries | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | U.S. treasuries | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	4,390,568	3,689,286
Fair Value, Measurements, Recurring | Corporate | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Corporate | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	4,390,568	3,689,286
Fair Value, Measurements, Recurring | Corporate | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	395,149	361,684
Fair Value, Measurements, Recurring | Agencies | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Agencies | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	395,149	361,684
Fair Value, Measurements, Recurring | Agencies | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	383,838	549,613
Fair Value, Measurements, Recurring | Non-U.S. government | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-U.S. government | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	383,838	549,613
Fair Value, Measurements, Recurring | Non-U.S. government | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	710,429	955,301
Fair Value, Measurements, Recurring | Residential mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Residential mortgage-backed | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	710,429	955,301
Fair Value, Measurements, Recurring | Residential mortgage-backed | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	213,987	634,925
Fair Value, Measurements, Recurring | Commercial mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Commercial mortgage-backed | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	213,987	634,925
Fair Value, Measurements, Recurring | Commercial mortgage-backed | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	1,077,302	1,068,543
Fair Value, Measurements, Recurring | Asset-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Asset-backed | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	1,077,302	1,068,543
Fair Value, Measurements, Recurring | Asset-backed | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Catastrophe bonds | Fair Value, Inputs, Level 1, 2 and 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	1,241,468	1,104,034
Fair Value, Measurements, Recurring | Catastrophe bonds | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Catastrophe bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	1,241,468	1,104,034
Fair Value, Measurements, Recurring | Catastrophe bonds | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Term loans | Fair Value, Inputs, Level 1, 2 and 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	100,000	74,850
Fair Value, Measurements, Recurring | Term loans | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0
Fair Value, Measurements, Recurring | Term loans | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0
Fair Value, Measurements, Recurring | Term loans | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	100,000	74,850
Fair Value, Measurements, Recurring | Direct private equity investments | Fair Value, Inputs, Level 1, 2 and 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	66,780	88,373
Fair Value, Measurements, Recurring | Direct private equity investments | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Direct private equity investments | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Direct private equity investments | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	66,780	88,373
Fair Value, Measurements, Recurring | Fund investments | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	1,086,706	725,802
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Significant Unobservable Inputs (Level 3) | Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	3,500	6,100
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Significant Unobservable Inputs (Level 3) | Other liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other liabilities	$ (5,300)	$ (10,800)