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, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	$ 46,425.2	[1]
Amount Shown on the Balance Sheet	51,130.9
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	1,202.8	[1]
Fair Value of Investments	1,382.4
Amount Shown on the Balance Sheet	1,382.4
States, Municipalities, and Political Subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,868.0	[1]
Fair Value of Investments	2,158.0
Amount Shown on the Balance Sheet	2,158.0
Foreign Governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	714.8	[1]
Fair Value of Investments	914.7
Amount Shown on the Balance Sheet	914.7
Public Utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	6,916.1	[1]
Fair Value of Investments	8,022.7
Amount Shown on the Balance Sheet	8,022.7
Mortgage/Asset-backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	2,104.9	[1]
Fair Value of Investments	2,230.4
Amount Shown on the Balance Sheet	2,230.4
All Other Corporate Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	26,707.1	[1]
Fair Value of Investments	29,467.2
Amount Shown on the Balance Sheet	29,467.2
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	41.9
Amount Shown on the Balance Sheet	41.9
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	39,552.7	[1]
Fair Value of Investments	44,217.3
Amount Shown on the Balance Sheet	44,217.3
Mortgages
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	2,038.9	[1]
Amount Shown on the Balance Sheet	2,038.9
Policy Loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	3,463.2	[1]
Amount Shown on the Balance Sheet	3,463.2
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	32.7	[2]
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1.1	[1]
Amount Shown on the Balance Sheet	1.2
Miscellaneous Long-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	589.3	[1]
Amount Shown on the Balance Sheet	597.6	[3]
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	780.0	[1]
Amount Shown on the Balance Sheet	$ 780.0

[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.