Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost, fixed maturities	$ 70,263.1	$ 72,869.9
Gross unrealized gains, fixed maturities	512.6	5,708.6
Gross unrealized losses, fixed maturities	7,878.0	408.5
Allowance, fixed maturities	7.8	15.5	$ 7.4
Fair value, fixed maturities	62,889.9	78,154.5
Accrued interest receivable	578.0	542.6
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	1,990.9	1,978.0
Gross unrealized gains, fixed maturities	0.1	148.0
Gross unrealized losses, fixed maturities	251.2	37.4
Fair value, fixed maturities	1,739.8	2,088.6
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	611.2	851.0
Gross unrealized gains, fixed maturities	20.9	133.1
Gross unrealized losses, fixed maturities	64.8	2.1
Fair value, fixed maturities	567.3	982.0
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	7,355.4	8,290.7
Gross unrealized gains, fixed maturities	13.7	1,030.3
Gross unrealized losses, fixed maturities	1,136.8	16.6
Fair value, fixed maturities	6,232.3	9,304.4
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	40,370.4	42,139.2
Gross unrealized gains, fixed maturities	461.0	4,044.8
Gross unrealized losses, fixed maturities	4,640.5	224.5
Allowance, fixed maturities	7.7	15.1	0.9
Fair value, fixed maturities	36,183.2	45,944.4
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	2,420.6	3,122.3
Gross unrealized gains, fixed maturities	6.2	59.0
Gross unrealized losses, fixed maturities	198.1	28.4
Fair value, fixed maturities	2,228.7	3,152.9
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,572.2	5,436.2
Gross unrealized gains, fixed maturities	0.5	157.8
Gross unrealized losses, fixed maturities	708.1	31.5
Allowance, fixed maturities		0.3	4.3
Fair value, fixed maturities	4,864.6	5,562.2
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	4,705.6	3,564.7
Gross unrealized gains, fixed maturities	4.5	4.5
Gross unrealized losses, fixed maturities	143.7	9.6
Allowance, fixed maturities			$ 2.2
Fair value, fixed maturities	4,566.4	3,559.6
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	7,236.8	7,487.8
Gross unrealized gains, fixed maturities	5.7	131.1
Gross unrealized losses, fixed maturities	734.8	58.4
Allowance, fixed maturities	0.1	0.1
Fair value, fixed maturities	$ 6,507.6	$ 7,560.4

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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