Investments - Debt Maturities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	$ 2,538.8
Without single maturity date, Fair Value	2,806.2
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	46.20%	41.80%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	482.9
Without single maturity date, Fair Value	498.9
Fixed maturities, including securities pledged, Amortized Cost	482.9	475.7
Fixed maturities, including securities pledged, Fair Value	498.9	495.6
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	570.1
One year or less, Fair Value	591.4
After one year through five years, Amortized Cost	4,171.7
After one year through five years, Fair Value	4,437.9
After five years through ten years, Amortized Cost	6,281.1
After five years through ten years, Fair Value	6,462.9
After ten years, Amortized Cost	5,756.2
After ten years, Fair Value	5,921.8
Fixed maturities, including securities pledged, Amortized Cost	19,800.8	19,210.6
Fixed maturities, including securities pledged, Fair Value	$ 20,719.1	$ 21,455.2