Fair Value Measurements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 5,789,968
Equity securities	14,636		$ 14,771
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,789,968		5,723,113
Equity securities	14,636		14,771
Cash equivalents (1)	283,185		367,517
Total assets	6,087,789		6,105,401
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	212,375		95,828
Equity securities	14,636		14,771
Cash equivalents (1)	203,570		367,301
Total assets	430,581		477,900
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,577,593		5,627,285
Equity securities	0		0
Cash equivalents (1)	79,615		216
Total assets	5,657,208		5,627,501
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative related to Home Re transactions	100		2,400
Significant Unobservable Inputs (Level 3) | Real Estate Acquired
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases of real estate assets acquired	2,300	$ 100
Sales of real estate assets acquired	500	$ 1,200
Total fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,789,968		5,723,113
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	263,913		161,682
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	263,913		161,682
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	212,375		95,828
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	51,538		65,854
Obligations of U.S. states and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,873,380		1,908,078
Obligations of U.S. states and political subdivisions | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,873,380		1,908,078
Obligations of U.S. states and political subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Obligations of U.S. states and political subdivisions | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,873,380		1,908,078
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,531,941		2,515,581
Corporate debt securities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,531,941		2,515,581
Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Corporate debt securities | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,531,941		2,515,581
ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	173,872		171,273
ABS | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	173,872		171,273
ABS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
ABS | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	173,872		171,273
RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	358,317		330,773
RMBS | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	358,317		330,773
RMBS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
RMBS | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	358,317		330,773
CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	262,826		277,457
CMBS | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	262,826		277,457
CMBS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
CMBS | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	262,826		277,457
CLOs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	291,728		326,096
CLOs | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	291,728		326,096
CLOs | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
CLOs | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	291,728		326,096
Foreign government debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,770		3,843
Foreign government debt | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,770		3,843
Foreign government debt | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Foreign government debt | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,770		3,843
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	30,221		28,330
Commercial paper | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	30,221		28,330
Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Commercial paper | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 30,221		$ 28,330