Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 5,564,053		$ 5,409,698
Equity securities	14,523		14,140
Real Estate Acquired
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases of real estate assets acquired	100	$ 500
Sales of real estate assets acquired	1,200	$ 1,000
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	14,523		14,140
Cash equivalents	362,788		328,756
Total assets	5,941,364		5,752,594
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]
Equity securities	14,523		14,140
Cash equivalents	328,906		324,129
Total assets	516,690		455,166
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0		0
Cash equivalents	33,882		4,627
Total assets	5,424,674		5,297,428
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	(1,500)		2,500
Total fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,564,053		5,409,698
Total fixed income securities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,564,053		5,409,698
Total fixed income 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	173,261		116,897
Total fixed income 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	5,390,792		5,292,801
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	215,746		135,900
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	215,746		135,900
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	173,261		116,897
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	42,485		19,003
Obligations of U.S. states and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,212,340		2,149,054
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	2,212,340		2,149,054
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	2,212,340		2,149,054
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,177,942		2,221,141
Corporate debt securities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,177,942		2,221,141
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	2,177,942		2,221,141
ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	119,371		120,687
ABS | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	119,371		120,687
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	119,371		120,687
RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	195,505		198,009
RMBS | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	195,505		198,009
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	195,505		198,009
CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	231,905		237,216
CMBS | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	231,905		237,216
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	231,905		237,216
CLOs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	330,574		329,832
CLOs | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	330,574		329,832
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	330,574		329,832
Foreign government debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,827		3,787
Foreign government debt | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,827		3,787
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	3,827		3,787
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	76,843		14,072
Commercial paper | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	76,843		14,072
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	$ 76,843		$ 14,072