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Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2023 | Dec. 31, 2022
Percent of Total Fair Value |  |
Due in 1 year Or less (percent) | 5.60% | 4.40%
Due after 1 year through 5 years (percent) | 27.10% | 26.30%
Due after 5 years through 10 years (percent) | 27.40% | 27.90%
Due after 10 years through 20 years (percent) | 23.90% | 25.00%
Due after 20 years (percent) | 16.00% | 16.40%
Total (percent) | 100.00% | 100.00%
Fair Value |  |
Due in 1 year or less | $ 278.3 |
Due after 1 year through 5 years | 1,343.4 |
Due after 5 years through 10 years | 1,357.3 |
Due after 10 years through 20 years | 1,187.1 |
Due after 20 years | 795.5 |
Total | 4,961.6 | $ 5,185.0
Amortized Cost, net |  |
Due in 1 year or less | 287.8 |
Due after 1 year through 5 years | 1,420.7 |
Due after 5 years through 10 years | 1,480.7 |
Due after 10 years through 20 years | 1,417.8 |
Due after 20 years | 1,034.6 |
Amortized Cost, net | $ 5,641.6 | $ 5,756.9
Average option-adjusted duration, in years | 6 years 1 month 6 days | 6 years 4 months 24 days

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