Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost, fixed maturities	$ 71,714.6	$ 72,869.9
Gross unrealized gains, fixed maturities	1,970.8	5,708.6
Gross unrealized losses, fixed maturities	2,251.4	408.5
Allowance, fixed maturities	20.4	15.5	$ 10.5	$ 7.4
Fair value, fixed maturities	71,413.6	78,154.5
Accrued interest receivable	573.5	542.6
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	1,933.7	1,978.0
Gross unrealized gains, fixed maturities	12.8	148.0
Gross unrealized losses, fixed maturities	76.7	37.4
Fair value, fixed maturities	1,869.8	2,088.6
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	836.4	851.0
Gross unrealized gains, fixed maturities	79.1	133.1
Gross unrealized losses, fixed maturities	13.8	2.1
Fair value, fixed maturities	901.7	982.0
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	8,191.6	8,290.7
Gross unrealized gains, fixed maturities	309.0	1,030.3
Gross unrealized losses, fixed maturities	266.5	16.6
Fair value, fixed maturities	8,234.1	9,304.4
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	42,197.1	42,139.2
Gross unrealized gains, fixed maturities	1,511.5	4,044.8
Gross unrealized losses, fixed maturities	1,276.8	224.5
Allowance, fixed maturities	20.0	15.1	0.9	0.9
Fair value, fixed maturities	42,411.8	45,944.4
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	1,978.3	3,122.3
Gross unrealized gains, fixed maturities	10.0	59.0
Gross unrealized losses, fixed maturities	82.9	28.4
Fair value, fixed maturities	1,905.4	3,152.9
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,625.1	5,436.2
Gross unrealized gains, fixed maturities	15.0	157.8
Gross unrealized losses, fixed maturities	216.6	31.5
Allowance, fixed maturities	0.3	0.3	7.0	4.3
Fair value, fixed maturities	5,423.2	5,562.2
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	3,919.3	3,564.7
Gross unrealized gains, fixed maturities	2.9	4.5
Gross unrealized losses, fixed maturities	33.6	9.6
Allowance, fixed maturities			$ 2.6	$ 2.2
Fair value, fixed maturities	3,888.6	3,559.6
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	7,033.1	7,487.8
Gross unrealized gains, fixed maturities	30.5	131.1
Gross unrealized losses, fixed maturities	284.5	58.4
Allowance, fixed maturities	0.1	0.1
Fair value, fixed maturities	$ 6,779.0	$ 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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