Investments Debt Maturities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 551.5
One year or less, Fair Value	579.7
After one year through five years, Amortized Cost	3,910.9
After one year through five years, Fair Value	4,154.1
After five years through ten years, Amortized Cost	5,291.1
After five years through ten years, Fair Value	5,796.3
After ten years, Amortized Cost	5,026.0
After ten years, Fair Value	5,736.1
Fixed maturities, including securities pledged, Fair Value	19,900.0
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	2,794.4
Without single maturity date, Fair Value	3,149.1
Percent of CMO holdings invested interest only or principal only types of CMOs	43.70%	41.10%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	451.3
Without single maturity date, Fair Value	456.4
Fixed maturities, including securities pledged, Amortized Cost	451.3	441.5
Fixed maturities, including securities pledged, Fair Value	456.4	438.8
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	18,025.2	17,662.3
Fixed maturities, including securities pledged, Fair Value	$ 19,871.7	19,240.2