Investments Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 391.9	$ 1,547.7
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	7.8	50.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	87.8	598.1
More Than Six Months and Twelve Months or Less Below Amortized Cost	6.4	42.3
More Than Twelve Months Below Amortized Cost, Fair Value	685.8	1,030.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	114.3	205.8
Total Fair Value	1,165.5	3,176.7
Total Unrealized Capital Losses	128.5	299.0
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	85.70%	83.40%
U.S. corporate, state and municipalities
Schedule of Available-for-sale Securities [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	255.8	798.9
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	2.3	17.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	55.2	97.6
More Than Six Months and Twelve Months or Less Below Amortized Cost	3.8	4.1
More Than Twelve Months Below Amortized Cost, Fair Value	132.1	208.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	17.2	20.6
Total Fair Value	443.1	1,104.5
Total Unrealized Capital Losses	23.3	42.3
Foreign
Schedule of Available-for-sale Securities [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	79.8	476.5
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	4.1	30.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	10.0	51.1
More Than Six Months and Twelve Months or Less Below Amortized Cost	1.5	5.0
More Than Twelve Months Below Amortized Cost, Fair Value	161.1	339.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	22.3	34.6
Total Fair Value	250.9	867.1
Total Unrealized Capital Losses	27.9	69.8
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	26.2	74.6
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	0.6	0.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	21.3	188.2
More Than Six Months and Twelve Months or Less Below Amortized Cost	0.6	5.7
More Than Twelve Months Below Amortized Cost, Fair Value	219.6	305.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	35.4	84.3
Total Fair Value	267.1	568.4
Total Unrealized Capital Losses	36.6	90.9
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	26.9	155.1
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	0.8	1.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1.3	234.7
More Than Six Months and Twelve Months or Less Below Amortized Cost	0.5	17.9
More Than Twelve Months Below Amortized Cost, Fair Value	16.6	35.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	3.2	6.6
Total Fair Value	44.8	425.5
Total Unrealized Capital Losses	4.5	26.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	3.2	42.6
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	0	0.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0	26.5
More Than Six Months and Twelve Months or Less Below Amortized Cost	0	9.6
More Than Twelve Months Below Amortized Cost, Fair Value	156.4	142.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	36.2	59.7
Total Fair Value	159.6	211.2
Total Unrealized Capital Losses	$ 36.2	$ 69.6