Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 2,494,954,000	$ 1,993,059,000
Other liabilities	(5,331,000)	(10,827,000)
Carrying Value	1,170,442,000	1,168,353,000
Fair Value	1,100,000,000	1,300,000,000
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	(181,300,000)	41,700,000	$ (4,700,000)
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	2,494,954,000	1,993,059,000
Other liabilities		(3,792,000)
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	166,780,000	163,223,000
Other liabilities	(1,832,000)	(4,727,000)
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	(1,832,000)	(4,727,000)
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	66,780,000	88,373,000
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	66,780,000	88,373,000
Significant Unobservable Inputs (Level 3) | Term loans | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	100,000,000	74,850,000
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	100,000,000	74,900,000
Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 1,086,706,000	$ 725,803,000
U.S. treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	4.30%	1.10%
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	6.30%	2.60%
Agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	4.60%	1.20%
Non-U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	4.70%	1.20%
Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	5.40%	2.20%
Weighted average life	8 years 7 months 6 days	5 years 7 months 6 days
Commercial mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	7.40%	1.90%
Weighted average life	3 years 2 months 12 days	4 years 1 month 6 days
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	7.40%	1.80%
Weighted average life	5 years 2 months 12 days	5 years 4 months 24 days
Short term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average yield to maturity	4.10%	0.10%
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
Private credit funds | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 771,383,000	$ 473,112,000
Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 66,780,000	88,373,000
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
Direct private equity investments | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 315,323,000	241,297,000
Hedge funds | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments fair value	$ 0	$ 11,394,000