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Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020
Available-for-sale securities
Amortized cost, fixed maturities	$ 70,811.1		$ 70,546.8
Gross unrealized gains, fixed maturities	6,106.5		8,380.0
Gross unrealized losses, fixed maturities	344.2		209.1
Allowance, fixed maturities	16.3	$ 8.7	7.4	$ 12.1	$ 7.2
Fair value, fixed maturities	76,557.1		78,710.3
Accrued interest receivable	570.1		552.5
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	1,821.7		1,893.1
Gross unrealized gains, fixed maturities	140.6		228.9
Gross unrealized losses, fixed maturities	40.5		10.5
Fair value, fixed maturities	1,921.8		2,111.5
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	906.3		881.6
Gross unrealized gains, fixed maturities	139.3		192.1
Gross unrealized losses, fixed maturities	0.8
Fair value, fixed maturities	1,044.8		1,073.7
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	8,255.6		8,004.9
Gross unrealized gains, fixed maturities	1,004.3		1,175.5
Gross unrealized losses, fixed maturities	26.9		12.6
Fair value, fixed maturities	9,233.0		9,167.8
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	41,412.9		41,289.9
Gross unrealized gains, fixed maturities	4,350.9		6,160.9
Gross unrealized losses, fixed maturities	203.0		95.1
Allowance, fixed maturities	15.9	8.5	0.9	6.2	2.8
Fair value, fixed maturities	45,544.9		47,354.8
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	2,816.2		2,857.6
Gross unrealized gains, fixed maturities	82.8		129.4
Gross unrealized losses, fixed maturities	16.4		0.2
Fair value, fixed maturities	2,882.6		2,986.8
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,086.5		4,741.2
Gross unrealized gains, fixed maturities	219.2		241.3
Gross unrealized losses, fixed maturities	22.2		35.9
Allowance, fixed maturities	0.3	$ 0.2	4.3	4.0	3.4
Fair value, fixed maturities	5,283.2		4,942.3
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	3,569.6		4,045.9
Gross unrealized gains, fixed maturities	7.3		8.7
Gross unrealized losses, fixed maturities	10.6		24.9
Allowance, fixed maturities			2.2	$ 1.9	$ 1.0
Fair value, fixed maturities	3,566.3		4,027.5
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	6,942.3		6,832.6
Gross unrealized gains, fixed maturities	162.1		243.2
Gross unrealized losses, fixed maturities	23.8		29.9
Allowance, fixed maturities	0.1
Fair value, fixed maturities	$ 7,080.5		$ 7,045.9