Schedule I - Summary of Investments, Other Than Investments in Related Parties Schedule I - Summary of Investments, Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2022 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 46,452.1	[1]
Amount Shown on the Balance Sheet	43,712.3
Commercial Real Estate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amount Shown on the Balance Sheet	2,435.4
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	503.8	[1]
Fair Value of Investments	498.2
Amount Shown on the Balance Sheet	498.2
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	4,006.0	[1]
Fair Value of Investments	3,457.2
Amount Shown on the Balance Sheet	3,457.2
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	908.1	[1]
Fair Value of Investments	827.1
Amount Shown on the Balance Sheet	827.1
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	5,170.9	[1]
Fair Value of Investments	4,956.9
Amount Shown on the Balance Sheet	4,956.9
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	592.1	[1]
Fair Value of Investments	573.3
Amount Shown on the Balance Sheet	573.3
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	26,640.3	[1]
Fair Value of Investments	24,524.6
Amount Shown on the Balance Sheet	24,524.6
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	4.0	[1]
Fair Value of Investments	3.5
Amount Shown on the Balance Sheet	3.5
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	37,825.2	[1]
Fair Value of Investments	34,840.8
Amount Shown on the Balance Sheet	34,840.8
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	3,601.2	[1]
Amount Shown on the Balance Sheet	3,601.2
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	[1]
Amount Shown on the Balance Sheet	89.1	[2]
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	23.1	[1]
Amount Shown on the Balance Sheet	25.8	[3]
Private Equity Partnerships
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	1,032.2
Amount Shown on the Balance Sheet	1,194.3	[3]
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	130.9	[1]
Amount Shown on the Balance Sheet	130.9	[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	1,394.8	[1]
Amount Shown on the Balance Sheet	$ 1,394.8

[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 expected credit losses.

[2] Derivatives are carried at fair value.

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

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