Investments Debt Maturities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 864.0
One year or less, Fair Value	904.5
After one year through five years, Amortized Cost	3,786.9
After one year through five years, Fair Value	4,074.0
After five years through ten years, Amortized Cost	5,248.3
After five years through ten years, Fair Value	5,892.1
After ten years, Amortized Cost	5,249.9
After ten years, Fair Value	6,149.7
Fixed maturities, including securities pledged, Fair Value	20,900.0
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Percent Collateralized Mortgage Backed Securities including Interest-Only Strip or Principal-Only Strip	41.10%	41.10%
Without single maturity date, Amortized Cost	2,999.8
Without single maturity date, Fair Value	3,388.9
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	477.7
Without single maturity date, Fair Value	493.6
Fixed maturities, including securities pledged, Amortized Cost	477.7	441.5
Fixed maturities, including securities pledged, Fair Value	493.6	438.8
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	18,626.6	17,662.3
Fixed maturities, including securities pledged, Fair Value	$ 20,902.8	19,240.2