Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Available-for-sale securities
Amortized cost, fixed maturities	$ 71,413.8	$ 70,263.1
Gross unrealized gains, fixed maturities	626.5	512.6
Gross unrealized losses, fixed maturities	6,639.3	7,878.0
Allowance, fixed maturities	4.1	7.8	$ 20.4	$ 15.5
Fair value, fixed maturities	65,396.9	62,889.9
Accrued interest receivable	632.6	578.0
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	2,041.5	1,990.9
Gross unrealized gains, fixed maturities	1.5	0.1
Gross unrealized losses, fixed maturities	213.1	251.2
Fair value, fixed maturities	1,829.9	1,739.8
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	584.3	611.2
Gross unrealized gains, fixed maturities	20.9	20.9
Gross unrealized losses, fixed maturities	56.0	64.8
Fair value, fixed maturities	549.2	567.3
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	7,061.5	7,355.4
Gross unrealized gains, fixed maturities	21.5	13.7
Gross unrealized losses, fixed maturities	909.6	1,136.8
Fair value, fixed maturities	6,173.4	6,232.3
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	40,630.6	40,370.4
Gross unrealized gains, fixed maturities	542.6	461.0
Gross unrealized losses, fixed maturities	3,872.6	4,640.5
Allowance, fixed maturities	4.0	7.7	20.0	15.1
Fair value, fixed maturities	37,296.6	36,183.2
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	2,951.5	2,420.6
Gross unrealized gains, fixed maturities	16.3	6.2
Gross unrealized losses, fixed maturities	172.9	198.1
Fair value, fixed maturities	2,794.9	2,228.7
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,555.1	5,572.2
Gross unrealized gains, fixed maturities	1.2	0.5
Gross unrealized losses, fixed maturities	628.5	708.1
Allowance, fixed maturities			0.3	0.3
Fair value, fixed maturities	4,927.8	4,864.6
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	5,024.6	4,705.6
Gross unrealized gains, fixed maturities	8.8	4.5
Gross unrealized losses, fixed maturities	131.1	143.7
Fair value, fixed maturities	4,902.3	4,566.4
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	7,564.7	7,236.8
Gross unrealized gains, fixed maturities	13.7	5.7
Gross unrealized losses, fixed maturities	655.5	734.8
Allowance, fixed maturities	0.1	0.1	$ 0.1	$ 0.1
Fair value, fixed maturities	$ 6,922.8	$ 6,507.6

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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