Investments Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 290.4	$ 1,140.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	6.4	45.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	87.2	267.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	7.3	16.6
More Than Twelve Months Below Amortized Cost, Fair Value	322.1	450.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	49.4	91.3
Total, Fair Value	699.7	1,858.8
Total Unrealized Capital Losses	63.1	153.4
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	86.70%	83.20%
U.S. corporate, state and municipalities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	134.7	595.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.9	22.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	43.1	46.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.6	3.0
More Than Twelve Months Below Amortized Cost, Fair Value	38.4	52.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.5	5.3
Total, Fair Value	216.2	694.5
Total Unrealized Capital Losses	11.0	31.1
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	70.3	435.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4.1	19.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	28.8	49.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.2	4.6
More Than Twelve Months Below Amortized Cost, Fair Value	84.1	169.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	13.4	17.8
Total, Fair Value	183.2	654.7
Total Unrealized Capital Losses	19.7	41.5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	67.9	49.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.4	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	15.3	97.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.5	5.2
More Than Twelve Months Below Amortized Cost, Fair Value	150.9	175.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	20.2	46.1
Total, Fair Value	234.1	321.8
Total Unrealized Capital Losses	22.1	52.9
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	5.0	28.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	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	3.1	8.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.2	1.5
Total, Fair Value	8.1	106.2
Total Unrealized Capital Losses	0.2	5.8
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	12.5	32.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0	4.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	1.3
More Than Twelve Months Below Amortized Cost, Fair Value	45.6	44.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	10.1	20.6
Total, Fair Value	58.1	81.6
Total Unrealized Capital Losses	$ 10.1	$ 22.1