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, 2024	Dec. 31, 2023
Available-for-sale securities, Amortized cost
Due in one year or less	$ 49,077
Due after one year through five years	453,677
Due after five years through ten years	186,695
Due after ten years	372,256
Amortized cost	1,061,705
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	48,447
Due after one year through five years	456,863
Due after five years through ten years	188,732
Due after ten years	362,950
Estimated fair value	1,056,992
Held-to-maturity securities, Estimated fair value
Held-to-maturity securities - due after ten years	4,833
Amortized cost	1,066,538
Estimated fair value	1,061,825
Fair value of loaned securities	$ 8,135	$ 0