Investments - Debt Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 756.4
One year or less, Fair Value	769.3
After one year through five years, Amortized Cost	4,729.8
After one year through five years, Fair Value	4,958.4
After five years through ten years, Amortized Cost	6,057.2
After five years through ten years, Fair Value	6,293.7
After ten years, Amortized Cost	7,761.8
After ten years, Fair Value	8,565.2
Fixed maturities, including securities pledged, Amortized Cost	23,849.1	$ 24,014.8
Fixed maturities, including securities pledged, Fair Value	25,296.3	$ 24,895.0
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	3,975.7
Without single maturity date, Fair Value	$ 4,131.3
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	53.10%	58.40%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	$ 568.2
Without single maturity date, Fair Value	578.4
Fixed maturities, including securities pledged, Amortized Cost	568.2	$ 466.4
Fixed maturities, including securities pledged, Fair Value	$ 578.4	$ 475.7