Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 1,603.7
Due after one year through five years	9,104.5
Due after five years through ten years	11,055.5
Due after ten years	28,554.2
Subtotal	50,317.9
Mortgage-backed and other asset-backed securities	21,095.9
Amortized cost, fixed maturities	71,413.8	$ 70,263.1
Fair value of fixed maturities available-for-sale
Due in one year or less	1,593.3
Due after one year through five years	8,819.0
Due after five years through ten years	10,268.5
Due after ten years	25,168.3
Subtotal	45,849.1
Mortgage-backed and other asset-backed securities	19,547.8
Fair value, fixed maturities	$ 65,396.9	$ 62,889.9