Investments Debt Maturities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Available-for-sale Securities, Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 1,149.4
One year or less, Fair Value	1,178.3
After one year through five years, Amortized Cost	4,807.7
After one year through five years, Fair Value	5,077.6
After ten years, Amortized Cost	5,984.8
After ten years, Fair Value	6,611.9
After ten years, Amortized Cost	3,886.0
After ten years, Fair Value	4,585.7
Mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Securities without single maturity date, Amortized Cost	3,380.9
Securities without single maturity date, Fair Value	3,825.0
Percent of CMO holdings invested interest only or principal only types of CMOs	32.30%	28.30%
Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Securities without single maturity date, Amortized Cost	706.1
Securities without single maturity date, Fair Value	689.8
Fixed maturities, including securities pledged, Amortized Cost	706.1	734.3
Fixed maturities, including securities pledged, Fair Value	689.8	673.6
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	19,914.9	21,362.4
Fixed maturities, including securities pledged, Fair Value	$ 21,968.3	22,748.5