Investments - Debt Maturities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 800.6
One year or less, Fair Value	823.4
After one year through five years, Amortized Cost	3,905.6
After one year through five years, Fair Value	4,142.0
After five years through ten years, Amortized Cost	6,152.3
After five years through ten years, Fair Value	6,365.7
After ten years, Amortized Cost	5,654.2
After ten years, Fair Value	5,896.7
Fixed maturities, including securities pledged, Amortized Cost	19,519.6	19,210.6
Fixed maturities, including securities pledged, Fair Value	20,530.4	21,455.2
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	2,527.7
Without single maturity date, Fair Value	2,805.7
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	43.10%	41.80%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	479.2
Without single maturity date, Fair Value	496.9
Fixed maturities, including securities pledged, Amortized Cost	479.2	475.7
Fixed maturities, including securities pledged, Fair Value	$ 496.9	$ 495.6