Schedule I - Summary of Investments, Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2024 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 48,032.6
Amount Shown on the Balance Sheet	45,706.3
Commercial Real Estate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,240.6	[1]
Amount Shown on the Balance Sheet	2,224.5
United States Government and Government Agencies and Authorities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	544.6	[1]
Fair Value of Investments	530.5
Amount Shown on the Balance Sheet	530.5
States, Municipalities, and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	3,795.6	[1]
Fair Value of Investments	3,291.4
Amount Shown on the Balance Sheet	3,291.4
Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	912.1	[1]
Fair Value of Investments	768.1
Amount Shown on the Balance Sheet	768.1
Public Utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	5,525.0	[1]
Fair Value of Investments	5,292.9
Amount Shown on the Balance Sheet	5,292.9
Mortgage/Asset-backed Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	949.4	[1],[2]
Fair Value of Investments	917.2	[2]
Amount Shown on the Balance Sheet	917.2	[2]
All Other Corporate Bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	26,535.2	[1]
Fair Value of Investments	24,822.2
Amount Shown on the Balance Sheet	24,822.2
Redeemable Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	8.0	[1]
Fair Value of Investments	7.6
Amount Shown on the Balance Sheet	7.6
Fixed Maturities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	38,269.9	[1]
Fair Value of Investments	35,629.9
Amount Shown on the Balance Sheet	35,629.9
Policy Loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	3,617.2
Amount Shown on the Balance Sheet	3,617.2
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	0.0	[3]
Amount Shown on the Balance Sheet	90.9	[3]
Perpetual Preferred Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	20.5	[4]
Amount Shown on the Balance Sheet	24.5	[4]
Private Equity Funds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	1,215.7	[4]
Amount Shown on the Balance Sheet	1,450.6	[4]
Miscellaneous Long-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	128.4
Amount Shown on the Balance Sheet	128.4
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,540.3
Amount Shown on the Balance Sheet	$ 2,540.3

[1] The amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from declines in fair value, amortization of premiums, and/or accretion of discounts. The amortized cost for these investments does not include allowance for credit losses.

[2] Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types.

[3] Derivatives are carried at fair value.

[4] The difference between amortized cost and carrying value for private equity partnerships and perpetual preferred and equity securities primarily results from changes in the partnership owner's equity and the security's market valuation since acquisition, respectively.