Investments - Amortized Cost and Estimated Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Available-for-sale securities, Amortized cost
Due in one year or less	$ 63,952
Due after one year through five years	460,087
Due after five years through ten years	136,215
Due after ten years	404,962
Amortized cost	1,065,216
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	63,710
Due after one year through five years	465,927
Due after five years through ten years	138,111
Due after ten years	398,085
Estimated fair value	1,065,833
Held-to-maturity securities, Estimated fair value
Held-to-maturity securities - due after ten years	4,876
Amortized cost	1,070,049	$ 1,070,789
Estimated fair value	1,070,709	$ 1,048,549
Fair value of securities lent	$ 56,200