Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost, fixed maturities	$ 72,869.9	$ 70,546.8
Gross unrealized gains, fixed maturities	5,708.6	8,380.0
Gross unrealized losses, fixed maturities	408.5	209.1
Allowance, fixed maturities	15.5	7.4
Fair value, fixed maturities	78,154.5	78,710.3
Accrued interest receivable	542.6	552.5
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	1,978.0	1,893.1
Gross unrealized gains, fixed maturities	148.0	228.9
Gross unrealized losses, fixed maturities	37.4	10.5
Fair value, fixed maturities	2,088.6	2,111.5
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	851.0	881.6
Gross unrealized gains, fixed maturities	133.1	192.1
Gross unrealized losses, fixed maturities	2.1
Fair value, fixed maturities	982.0	1,073.7
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	8,290.7	8,004.9
Gross unrealized gains, fixed maturities	1,030.3	1,175.5
Gross unrealized losses, fixed maturities	16.6	12.6
Fair value, fixed maturities	9,304.4	9,167.8
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	42,139.2	41,289.9
Gross unrealized gains, fixed maturities	4,044.8	6,160.9
Gross unrealized losses, fixed maturities	224.5	95.1
Allowance, fixed maturities	15.1	0.9
Fair value, fixed maturities	45,944.4	47,354.8
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	3,122.3	2,857.6
Gross unrealized gains, fixed maturities	59.0	129.4
Gross unrealized losses, fixed maturities	28.4	0.2
Fair value, fixed maturities	3,152.9	2,986.8
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,436.2	4,741.2
Gross unrealized gains, fixed maturities	157.8	241.3
Gross unrealized losses, fixed maturities	31.5	35.9
Allowance, fixed maturities	0.3	4.3
Fair value, fixed maturities	5,562.2	4,942.3
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	3,564.7	4,045.9
Gross unrealized gains, fixed maturities	4.5	8.7
Gross unrealized losses, fixed maturities	9.6	24.9
Allowance, fixed maturities		2.2
Fair value, fixed maturities	3,559.6	4,027.5
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	7,487.8	6,832.6
Gross unrealized gains, fixed maturities	131.1	243.2
Gross unrealized losses, fixed maturities	58.4	29.9
Allowance, fixed maturities	0.1
Fair value, fixed maturities	$ 7,560.4	$ 7,045.9

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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