Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 3,515,566	$ 2,494,954
Other liabilities	(2,742)	(5,331)
Carrying Value	1,958,655	1,170,442
Fair Value	1,900,000	1,100,000
Other investments | Included in net investment income
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) included in net investment income	137,300	(181,200)	$ 41,700
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	3,515,566	2,494,954
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	157,563	166,780
Other liabilities	(515)	(1,832)
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Internal valuation model | Assumed reinsurance contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	(515)	(1,832)
Significant Unobservable Inputs (Level 3) | Direct private equity investments | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	59,905	66,780
Significant Unobservable Inputs (Level 3) | Direct private equity investments | Fair Value, Measurements, Recurring | Internal valuation model
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	59,905	66,780
Significant Unobservable Inputs (Level 3) | Term loans | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	97,658	100,000
Significant Unobservable Inputs (Level 3) | Term loans | Fair Value, Measurements, Recurring | Internal valuation model
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 97,700	$ 100,000
U.S. treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	4.10%	4.30%
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	5.70%	6.30%
Agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	4.60%	4.60%
Non-U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	4.40%	4.70%
Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	5.10%	5.40%
Weighted average life	7 years 8 months 12 days	8 years 7 months 6 days
Commercial mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	8.80%	7.40%
Weighted average life	2 years 2 months 12 days	3 years 2 months 12 days
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	7.00%	7.40%
Weighted average life	3 years 10 months 24 days	5 years 2 months 12 days
Short term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	5.30%	4.10%
Other Short Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	5.30%	0.40%
Private credit funds | Low
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	5 years
Private credit funds | High
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	10 years
Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 59,905	$ 66,780
Direct private equity investments | Low
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	5 years
Direct private equity investments | High
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidating period	10 years