Investments - Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 1,038.4	$ 1,140.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	13.7	45.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	39.1	267.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.7	16.6
More Than Twelve Months Below Amortized Cost, Fair Value	306.2	450.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	37.6	91.3
Total, Fair Value	1,383.7	1,858.8
Total Unrealized Capital Losses	53.0	153.4
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	89.10%	83.20%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	300.0	0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5	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	300.0	0
Total Unrealized Capital Losses	0.5	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	479.8	595.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	6.8	22.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	22.5	46.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.9	3.0
More Than Twelve Months Below Amortized Cost, Fair Value	49.4	52.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.4	5.3
Total, Fair Value	551.7	694.5
Total Unrealized Capital Losses	11.1	31.1
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	166.8	435.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4.7	19.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	7.8	49.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5	4.6
More Than Twelve Months Below Amortized Cost, Fair Value	87.7	169.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.2	17.8
Total, Fair Value	262.3	654.7
Total Unrealized Capital Losses	16.4	41.5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	68.7	49.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.6	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	7.2	97.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	5.2
More Than Twelve Months Below Amortized Cost, Fair Value	132.4	175.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	16.2	46.1
Total, Fair Value	208.3	321.8
Total Unrealized Capital Losses	18.1	52.9
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	7.5	28.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	1.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1.6	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	2.5	8.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.1	1.5
Total, Fair Value	11.6	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	15.6	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	34.2	44.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.7	20.6
Total, Fair Value	49.8	81.6
Total Unrealized Capital Losses	$ 6.7	$ 22.1