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-Topic 326
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Investments - Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost, fixed maturities	$ 70,781.4	$ 70,263.1
Gross unrealized gains, fixed maturities	773.6	512.6
Gross unrealized losses, fixed maturities	5,877.2	7,878.0
Allowance, fixed maturities	4.7	7.8	$ 15.5	$ 7.4
Fair value, fixed maturities	65,673.1	62,889.9
Accrued interest receivable	619.2	578.0
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	1,745.3	1,990.9
Gross unrealized gains, fixed maturities	19.3	0.1
Gross unrealized losses, fixed maturities	235.8	251.2
Fair value, fixed maturities	1,528.8	1,739.8
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	550.3	611.2
Gross unrealized gains, fixed maturities	17.3	20.9
Gross unrealized losses, fixed maturities	59.7	64.8
Fair value, fixed maturities	507.9	567.3
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	7,566.4	7,355.4
Gross unrealized gains, fixed maturities	44.3	13.7
Gross unrealized losses, fixed maturities	933.9	1,136.8
Fair value, fixed maturities	6,676.8	6,232.3
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	38,431.3	40,370.4
Gross unrealized gains, fixed maturities	578.5	461.0
Gross unrealized losses, fixed maturities	3,195.2	4,640.5
Allowance, fixed maturities	4.6	7.7	15.1	0.9
Fair value, fixed maturities	35,810.0	36,183.2
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	3,199.7	2,420.6
Gross unrealized gains, fixed maturities	26.0	6.2
Gross unrealized losses, fixed maturities	164.6	198.1
Fair value, fixed maturities	3,061.1	2,228.7
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,428.6	5,572.2
Gross unrealized gains, fixed maturities	1.2	0.5
Gross unrealized losses, fixed maturities	654.3	708.1
Allowance, fixed maturities			0.3	4.3
Fair value, fixed maturities	4,775.5	4,864.6
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	5,386.0	4,705.6
Gross unrealized gains, fixed maturities	44.9	4.5
Gross unrealized losses, fixed maturities	27.2	143.7
Allowance, fixed maturities				$ 2.2
Fair value, fixed maturities	5,403.7	4,566.4
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	8,473.8	7,236.8
Gross unrealized gains, fixed maturities	42.1	5.7
Gross unrealized losses, fixed maturities	606.5	734.8
Allowance, fixed maturities	0.1	0.1	$ 0.1
Fair value, fixed maturities	$ 7,909.3	$ 6,507.6