Investments - Debt Maturities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 853.5
One year or less, Fair Value	880.9
After one year through five years, Amortized Cost	3,953.8
After one year through five years, Fair Value	4,249.9
After five years through ten years, Amortized Cost	5,700.3
After five years through ten years, Fair Value	6,339.8
After ten years, Amortized Cost	5,408.2
After ten years, Fair Value	6,292.4
Fixed maturities, including securities pledged, Fair Value	21,500.0	19,200.0
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	2,819.1
Without single maturity date, Fair Value	3,196.6
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	41.80%	41.10%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	475.7
Without single maturity date, Fair Value	495.6
Fixed maturities, including securities pledged, Amortized Cost	475.7	441.5
Fixed maturities, including securities pledged, Fair Value	495.6	438.8
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	19,210.6	17,662.3
Fixed maturities, including securities pledged, Fair Value	$ 21,455.2	$ 19,240.2