Investments - Amortized Cost and Estimated Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Available-for-sale securities, Amortized cost
Due in one year or less	$ 54,888
Due after one year through five years	588,721
Due after five years through ten years	205,037
Due after ten years	552,858
Amortized cost	1,401,504
Held-to-maturity securities, Amortized cost
Held-to-maturity securities - due after ten years	4,833
Available-for-sale securities, Estimated fair value
Due in one year or less	54,861
Due after one year through five years	588,118
Due after five years through ten years	203,455
Due after ten years	540,713
Estimated fair value	1,387,147
Held-to-maturity securities, Estimated fair value
Held-to-maturity securities - due after ten years	4,748
Amortized cost	1,406,337	$ 1,368,061
Estimated fair value	1,391,895	1,369,691
Fair value of securities lent	$ 55,300	$ 64,500