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, 2019 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 49,331.7	[1]
Amount Shown on the Balance Sheet	55,758.9
U.S. Government and Agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	1,246.1	[1]
Fair Value of Investments	1,401.7
Amount Shown on the Balance Sheet	1,401.7
State and Municipal Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,863.1	[1]
Fair Value of Investments	3,363.4
Amount Shown on the Balance Sheet	3,363.4
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	843.5	[1]
Fair Value of Investments	1,017.7
Amount Shown on the Balance Sheet	1,017.7
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	6,436.7	[1]
Fair Value of Investments	7,732.2
Amount Shown on the Balance Sheet	7,732.2
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	1,377.8	[1]
Fair Value of Investments	1,478.7
Amount Shown on the Balance Sheet	1,478.7
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	28,273.1	[1]
Fair Value of Investments	32,410.4
Amount Shown on the Balance Sheet	32,410.4
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	39.0	[1]
Fair Value of Investments	39.6
Amount Shown on the Balance Sheet	39.6
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	41,079.3	[1]
Fair Value of Investments	47,443.7
Amount Shown on the Balance Sheet	47,443.7
Mortgages
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,397.0	[1]
Amount Shown on the Balance Sheet	2,397.0
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	3,779.5	[1]
Amount Shown on the Balance Sheet	3,779.5
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	27.5	[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	32.6	[1]
Amount Shown on the Balance Sheet	32.6
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	748.8	[1]
Amount Shown on the Balance Sheet	784.1	[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	1,294.5	[1]
Amount Shown on the Balance Sheet	$ 1,294.5

[1] The amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and/or accretion of discounts.

[2] Derivatives are carried at fair value.

[3] The difference between amortized cost and carrying value primarily results from changes in the partnership owner's equity since acquisition.