Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 1,282.4
Due after one year through five years	9,308.1
Due after five years through ten years	10,066.3
Due after ten years	28,291.3
Subtotal	48,948.1
Mortgage-backed and other asset-backed securities	21,615.6
Amortized cost, fixed maturities	70,563.7	$ 70,263.1
Fair value of fixed maturities available-for-sale
Due in one year or less	1,265.7
Due after one year through five years	8,904.5
Due after five years through ten years	9,016.4
Due after ten years	22,691.5
Subtotal	41,878.1
Mortgage-backed and other asset-backed securities	19,703.9
Fair value, fixed maturities	$ 61,582.0	$ 62,889.9