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, 2021 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 46,037.8	[1]
Amount Shown on the Balance Sheet	52,150.3
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	460.1	[1]
Fair Value of Investments	580.1
Amount Shown on the Balance Sheet	580.1
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	4,150.2	[1]
Fair Value of Investments	4,727.5
Amount Shown on the Balance Sheet	4,727.5
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	952.0	[1]
Fair Value of Investments	1,146.6
Amount Shown on the Balance Sheet	1,146.6
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	5,266.4	[1]
Fair Value of Investments	6,416.0
Amount Shown on the Balance Sheet	6,416.0
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	587.9	[1]
Fair Value of Investments	638.3
Amount Shown on the Balance Sheet	638.3
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	25,966.1	[1]
Fair Value of Investments	29,823.4
Amount Shown on the Balance Sheet	29,823.4
Redeemable Preferred Stock
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	4.1
Amount Shown on the Balance Sheet	4.1
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	37,386.7	[1]
Fair Value of Investments	43,336.0
Amount Shown on the Balance Sheet	43,336.0
Mortgages
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,568.7	[1]
Amount Shown on the Balance Sheet	2,560.4
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	3,662.9	[1]
Amount Shown on the Balance Sheet	3,662.9
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	39.5	[2]
Equity Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	33.6	[1]
Amount Shown on the Balance Sheet	33.7	[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	846.7
Amount Shown on the Balance Sheet	978.6	[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	151.2	[1]
Amount Shown on the Balance Sheet	151.2	[3]
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	1,388.0	[1]
Amount Shown on the Balance Sheet	$ 1,388.0

[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 issuer's equity since acquisition, respectively.