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Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Available-for-sale securities
Amortized cost, fixed maturities	$ 70,563.7		$ 70,263.1
Gross unrealized gains, fixed maturities	262.5		512.6
Gross unrealized losses, fixed maturities	9,237.7		7,878.0
Allowance, fixed maturities	6.5	$ 8.5	7.8	$ 6.9	$ 12.5	$ 15.5
Fair value, fixed maturities	61,582.0		62,889.9
Accrued interest receivable	650.0		578.0
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	1,772.3		1,990.9
Gross unrealized gains, fixed maturities			0.1
Gross unrealized losses, fixed maturities	313.5		251.2
Fair value, fixed maturities	1,458.8		1,739.8
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	552.8		611.2
Gross unrealized gains, fixed maturities	12.9		20.9
Gross unrealized losses, fixed maturities	87.3		64.8
Fair value, fixed maturities	478.4		567.3
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	7,724.8		7,355.4
Gross unrealized gains, fixed maturities	1.7		13.7
Gross unrealized losses, fixed maturities	1,455.5		1,136.8
Fair value, fixed maturities	6,271.0		6,232.3
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	38,898.2		40,370.4
Gross unrealized gains, fixed maturities	200.6		461.0
Gross unrealized losses, fixed maturities	5,422.5		4,640.5
Allowance, fixed maturities	6.4	8.4	7.7	6.8	$ 12.5	15.1
Fair value, fixed maturities	33,669.9		36,183.2
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	2,978.7		2,420.6
Gross unrealized gains, fixed maturities	0.1		6.2
Gross unrealized losses, fixed maturities	295.1		198.1
Fair value, fixed maturities	2,683.7		2,228.7
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,438.8		5,572.2
Gross unrealized gains, fixed maturities			0.5
Gross unrealized losses, fixed maturities	779.2		708.1
Allowance, fixed maturities						0.3
Fair value, fixed maturities	4,659.6		4,864.6
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	5,135.3		4,705.6
Gross unrealized gains, fixed maturities	41.4		4.5
Gross unrealized losses, fixed maturities	42.9		143.7
Fair value, fixed maturities	5,133.8		4,566.4
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	8,062.8		7,236.8
Gross unrealized gains, fixed maturities	5.8		5.7
Gross unrealized losses, fixed maturities	841.7		734.8
Allowance, fixed maturities	0.1	$ 0.1	0.1	$ 0.1		$ 0.1
Fair value, fixed maturities	$ 7,226.8		$ 6,507.6