Investments Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 1,014.2	$ 1,140.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	21.6	45.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	223.2	267.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	9.8	16.6
More Than Twelve Months Below Amortized Cost, Fair Value	414.5	450.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	72.9	91.3
Total, Fair Value	1,651.9	1,858.8
Total Unrealized Capital Losses	104.3	153.4
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	85.00%	83.20%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	209.1	0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total, Fair Value	209.1	0
Total Unrealized Capital Losses	0	0
U.S. corporate, state and municipalities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	418.0	595.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	10.4	22.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	108.7	46.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	5.1	3.0
More Than Twelve Months Below Amortized Cost, Fair Value	46.5	52.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	8.4	5.3
Total, Fair Value	573.2	694.5
Total Unrealized Capital Losses	23.9	31.1
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	291.4	435.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	8.1	19.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	83.9	49.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.9	4.6
More Than Twelve Months Below Amortized Cost, Fair Value	77.7	169.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.0	17.8
Total, Fair Value	453.0	654.7
Total Unrealized Capital Losses	23.0	41.5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	49.1	49.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.6	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	25.3	97.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.7	5.2
More Than Twelve Months Below Amortized Cost, Fair Value	210.2	175.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	35.5	46.1
Total, Fair Value	284.6	321.8
Total Unrealized Capital Losses	38.8	52.9
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	41.5	28.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.4	1.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0	69.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	2.5
More Than Twelve Months Below Amortized Cost, Fair Value	30.5	8.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.0	1.5
Total, Fair Value	72.0	106.2
Total Unrealized Capital Losses	1.4	5.8
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	5.1	32.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	0.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	5.3	4.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	1.3
More Than Twelve Months Below Amortized Cost, Fair Value	49.6	44.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	17.0	20.6
Total, Fair Value	60.0	81.6
Total Unrealized Capital Losses	$ 17.2	$ 22.1