Schedule I - Summary of Investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 83,518.8
Value	91,336.4
Amount Shown on Consolidated Balance Sheet	90,833.8
U.S. Treasuries
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,279.0
Value	3,904.0
Amount Shown on Consolidated Balance Sheet	3,904.0
U.S. government agencies and authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	376.1
Value	435.9
Amount Shown on Consolidated Balance Sheet	435.9
State, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	659.5
Value	694.4
Amount Shown on Consolidated Balance Sheet	694.4
U.S. corporate securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	37,425.5
Value	40,740.8
Amount Shown on Consolidated Balance Sheet	40,740.8
Foreign
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	15,531.6
Value	16,444.2
Amount Shown on Consolidated Balance Sheet	16,444.2
Residential mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,972.7
Value	6,656.8
Amount Shown on Consolidated Balance Sheet	6,656.8
Commercial mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,916.3
Value	4,188.2
Amount Shown on Consolidated Balance Sheet	4,188.2
Other asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,538.1
Value	1,595.1
Amount Shown on Consolidated Balance Sheet	1,595.1
Fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	68,698.8
Value	74,659.4
Amount Shown on Consolidated Balance Sheet	74,659.4
Equity securities, available-for-sale
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	242.0
Value	271.8
Amount Shown on Consolidated Balance Sheet	271.8
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,711.4
Value	1,711.4
Amount Shown on Consolidated Balance Sheet	1,711.4
Mortgage loans on real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	9,794.1
Value	10,286.6
Amount Shown on Consolidated Balance Sheet	9,794.1
Policy loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,104.0
Value	2,104.0
Amount Shown on Consolidated Balance Sheet	2,104.0
Limited partnerships/corporations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	363.2
Value	363.2
Amount Shown on Consolidated Balance Sheet	363.2
Derivatives
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	495.0
Value	1,819.6
Amount Shown on Consolidated Balance Sheet	1,819.6
Other investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	110.3
Value	120.4
Amount Shown on Consolidated Balance Sheet	$ 110.3