Schedule I: Summary of Investments—Other Than Investments in Related Parties (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 6,407,842
Fair Value	5,805,630
Amount Reflected on the Consolidated Balance Sheet	5,805,630
Loaned securities	112,139	$ 103,996
Reinvested cash collateral held under securities lending agreements	99,735	$ 48,652
U.S. government and agency securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	174,138
Fair Value	145,442
Amount Reflected on the Consolidated Balance Sheet	145,442
State and municipal obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	164,325
Fair Value	142,386
Amount Reflected on the Consolidated Balance Sheet	142,386
Corporate bonds and notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	2,886,905
Fair Value	2,537,771
Amount Reflected on the Consolidated Balance Sheet	2,537,771
RMBS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1,025,795
Fair Value	928,399
Amount Reflected on the Consolidated Balance Sheet	928,399
CMBS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	645,890
Fair Value	593,357
Amount Reflected on the Consolidated Balance Sheet	593,357
CLO
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	518,677
Fair Value	498,192
Amount Reflected on the Consolidated Balance Sheet	498,192
Other ABS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	168,033
Fair Value	161,359
Amount Reflected on the Consolidated Balance Sheet	161,359
Foreign government and agency securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	5,118
Fair Value	4,975
Amount Reflected on the Consolidated Balance Sheet	4,975
Mortgage insurance-linked notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	54,578
Fair Value	53,019
Amount Reflected on the Consolidated Balance Sheet	53,019
Total fixed-maturities available for sale
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	5,643,459
Fair Value	5,064,900
Amount Reflected on the Consolidated Balance Sheet	5,064,900
Total fixed-maturities available for sale | Loaned securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Loaned securities	47,200
Trading securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	122,472
Fair Value	115,665
Amount Reflected on the Consolidated Balance Sheet	115,665
Reinvested cash collateral held under securities lending agreements	99,700
Common stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	230,525
Fair Value	213,519
Amount Reflected on the Consolidated Balance Sheet	213,519
Total equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	230,525
Fair Value	213,519
Amount Reflected on the Consolidated Balance Sheet	213,519
Mortgage loans held for sale
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	3,498
Fair Value	3,549
Amount Reflected on the Consolidated Balance Sheet	3,549
Other invested assets
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	5,336
Fair Value	5,511
Amount Reflected on the Consolidated Balance Sheet	5,511
Short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	402,552
Fair Value	402,486
Amount Reflected on the Consolidated Balance Sheet	402,486
Short-term investments | Loaned securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Loaned securities	400
Equity securities | Loaned securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Loaned securities	$ 64,600