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Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost, fixed maturities	$ 70,912.4		$ 72,869.9
Gross unrealized gains, fixed maturities	362.3		5,708.6
Gross unrealized losses, fixed maturities	8,858.7		408.5
Allowance, fixed maturities	6.9	$ 12.5	15.5	$ 16.3	$ 8.7	$ 7.4
Fair value, fixed maturities	62,409.1		78,154.5
Accrued interest receivable	594.2		542.6
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	2,300.1		1,978.0
Gross unrealized gains, fixed maturities			148.0
Gross unrealized losses, fixed maturities	254.6		37.4
Fair value, fixed maturities	2,045.5		2,088.6
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	635.1		851.0
Gross unrealized gains, fixed maturities	24.7		133.1
Gross unrealized losses, fixed maturities	84.3		2.1
Fair value, fixed maturities	575.5		982.0
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	7,598.0		8,290.7
Gross unrealized gains, fixed maturities	13.0		1,030.3
Gross unrealized losses, fixed maturities	1,194.3		16.6
Fair value, fixed maturities	6,416.7		9,304.4
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	41,897.2		42,139.2
Gross unrealized gains, fixed maturities	318.5		4,044.8
Gross unrealized losses, fixed maturities	5,598.8		224.5
Allowance, fixed maturities	6.8	$ 12.5	15.1	15.9	8.5	0.9
Fair value, fixed maturities	36,610.1		45,944.4
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	1,959.9		3,122.3
Gross unrealized gains, fixed maturities	0.9		59.0
Gross unrealized losses, fixed maturities	223.0		28.4
Fair value, fixed maturities	1,737.8		3,152.9
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,642.9		5,436.2
Gross unrealized gains, fixed maturities	1.4		157.8
Gross unrealized losses, fixed maturities	656.0		31.5
Allowance, fixed maturities			0.3	0.3	$ 0.2	4.3
Fair value, fixed maturities	4,988.3		5,562.2
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	4,128.9		3,564.7
Gross unrealized gains, fixed maturities	1.9		4.5
Gross unrealized losses, fixed maturities	136.1		9.6
Allowance, fixed maturities						$ 2.2
Fair value, fixed maturities	3,994.7		3,559.6
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	6,750.3		7,487.8
Gross unrealized gains, fixed maturities	1.9		131.1
Gross unrealized losses, fixed maturities	711.6		58.4
Allowance, fixed maturities	0.1		0.1	$ 0.1
Fair value, fixed maturities	$ 6,040.5		$ 7,560.4