Schedule I. Summary of Investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 23,979.1
Value	25,146.9
Amount Shown on Consolidated Balance Sheets	25,139.1
U.S. Treasuries
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	636.5
Value	670.1
Amount Shown on Consolidated Balance Sheets	670.1
U.S. government agencies and authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	237.1
Value	242.1
Amount Shown on Consolidated Balance Sheets	242.1
State, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	77.2
Value	83.0
Amount Shown on Consolidated Balance Sheets	83.0
U.S. corporate securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10,326.0
Value	10,668.2
Amount Shown on Consolidated Balance Sheets	10,668.2
Foreign securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,572.5	[1]
Value	5,770.6	[1]
Amount Shown on Consolidated Balance Sheets	5,770.6	[1]
Residential mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,916.3
Value	2,099.7
Amount Shown on Consolidated Balance Sheets	2,099.7
Commercial mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	624.5
Value	691.7
Amount Shown on Consolidated Balance Sheets	691.7
Other asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	465.8
Value	480.4
Amount Shown on Consolidated Balance Sheets	480.4
Fixed maturities pledged, included in securities pledged
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	19,855.9
Value	20,705.8
Amount Shown on Consolidated Balance Sheets	20,705.8
Equity securities, available-for-sale
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	119.4
Value	134.9
Amount Shown on Consolidated Balance Sheets	134.9
Mortgage loans on real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,396.1
Value	3,403.9
Amount Shown on Consolidated Balance Sheets	3,396.1
Policy loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	242.0
Value	242.0
Amount Shown on Consolidated Balance Sheets	242.0
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	15.0
Value	15.0
Amount Shown on Consolidated Balance Sheets	15.0
Limited partnerships/corporations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	180.9
Value	180.9
Amount Shown on Consolidated Balance Sheets	180.9
Derivatives
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	169.8
Value	464.4
Amount Shown on Consolidated Balance Sheets	$ 464.4

[1] The term "foreign" includes foreign governments, foreign political subdivisions, foreign public utilities and all other bonds of foreign issuers. Substantially all of the Company's foreign securities are denominated in U.S. dollars.