Schedule I: Summary of Investments—Other Than Investments in Related Parties (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 6,432,651
Allowance for Credit Losses	0	$ (948)
Fair Value	6,617,538
Amount Reflected on the Consolidated Balance Sheet	6,617,538
Reinvested cash collateral held under securities lending agreements	48,652	15,587
Loaned securities (Notes 5 and 6)	103,996	$ 57,499
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	221,407
Fair Value	221,730
Amount Reflected on the Consolidated Balance Sheet	221,730
State and municipal obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	162,964
Fair Value	177,257
Amount Reflected on the Consolidated Balance Sheet	177,257
Corporate bonds and notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	2,867,063
Fair Value	2,975,842
Amount Reflected on the Consolidated Balance Sheet	2,975,842
RMBS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	697,581
Fair Value	705,117
Amount Reflected on the Consolidated Balance Sheet	705,117
CMBS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	690,827
Fair Value	709,203
Amount Reflected on the Consolidated Balance Sheet	709,203
CLO
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	529,906
Fair Value	530,040
Amount Reflected on the Consolidated Balance Sheet	530,040
Other ABS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	210,657
Fair Value	211,187
Amount Reflected on the Consolidated Balance Sheet	211,187
Foreign government and agency securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	5,109
Fair Value	5,296
Amount Reflected on the Consolidated Balance Sheet	5,296
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	5,430,898
Fair Value	5,582,689
Amount Reflected on the Consolidated Balance Sheet	5,582,689
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 (Notes 5 and 6)	65,600
Trading securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	234,757
Fair Value	256,929
Amount Reflected on the Consolidated Balance Sheet	256,929
Reinvested cash collateral held under securities lending agreements	48,700
Common stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	211,905
Fair Value	222,247
Amount Reflected on the Consolidated Balance Sheet	222,247
Total equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	211,905
Fair Value	222,247
Amount Reflected on the Consolidated Balance Sheet	222,247
Short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	551,516
Fair Value	551,508
Amount Reflected on the Consolidated Balance Sheet	551,508
Short-term investments | Loaned securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Loaned securities (Notes 5 and 6)	400
Other invested assets
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	3,575
Fair Value	4,165
Amount Reflected on the Consolidated Balance Sheet	4,165
Equity securities | Loaned securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Loaned securities (Notes 5 and 6)	38,000
Mortgage insurance-linked notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	45,384
Fair Value	47,017
Amount Reflected on the Consolidated Balance Sheet	$ 47,017