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, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	$ 47,332.7	[1]
Amount Shown on the Balance Sheet	53,044.0
United States Government and Government Agencies and Authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,311.1	[1]
Fair Value of Investments	1,482.5
Amount Shown on the Balance Sheet	1,482.5
States, Municipalities, and Political Subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,942.8	[1]
Fair Value of Investments	2,336.9
Amount Shown on the Balance Sheet	2,336.9
Foreign Governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	673.0	[1]
Fair Value of Investments	863.9
Amount Shown on the Balance Sheet	863.9
Public Utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	6,952.7	[1]
Fair Value of Investments	8,236.5
Amount Shown on the Balance Sheet	8,236.5
Mortgage/Asset-backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,873.2	[1]
Fair Value of Investments	1,973.6
Amount Shown on the Balance Sheet	1,973.6
All Other Corporate Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	26,988.7	[1]
Fair Value of Investments	30,522.4
Amount Shown on the Balance Sheet	30,522.4
Redeemable Preferred Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	39.0	[1]
Fair Value of Investments	42.0
Amount Shown on the Balance Sheet	42.0
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	39,780.5	[1]
Fair Value of Investments	45,457.8
Amount Shown on the Balance Sheet	45,457.8
Mortgages
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	2,213.2	[1]
Amount Shown on the Balance Sheet	2,213.2
Policy Loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	3,571.1	[1]
Amount Shown on the Balance Sheet	3,571.1
Derivatives
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	0.0	[1]
Amount Shown on the Balance Sheet	19.5	[2]
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	11.1	[1]
Amount Shown on the Balance Sheet	11.7
Miscellaneous Long-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	601.7	[1]
Amount Shown on the Balance Sheet	615.6	[3]
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,155.1	[1]
Amount Shown on the Balance Sheet	$ 1,155.1

[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.