Investments - Debt Maturities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	$ 2,885.5
Without single maturity date, Fair Value	3,197.6
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	58.70%	57.30%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	337.1
Without single maturity date, Fair Value	349.9
Fixed maturities, including securities pledged, Amortized Cost	337.1	389.0
Fixed maturities, including securities pledged, Fair Value	349.9	400.4
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	567.7
One year or less, Fair Value	574.5
After one year through five years, Amortized Cost	4,261.7
After one year through five years, Fair Value	4,522.1
After five years through ten years, Amortized Cost	5,950.9
After five years through ten years, Fair Value	6,314.4
After ten years, Amortized Cost	6,360.6
After ten years, Fair Value	7,263.6
Fixed maturities, including securities pledged, Amortized Cost	20,363.5	20,035.1
Fixed maturities, including securities pledged, Fair Value	$ 22,222.1	21,616.6