Investments - Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 4,989.2	$ 1,038.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	263.7	13.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	343.0	39.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	37.6	1.7
More Than Twelve Months Below Amortized Cost, Fair Value	184.7	306.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	25.7	37.6
Total, Fair Value	5,516.9	1,383.7
Total Unrealized Capital Losses	327.0	53.0
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	87.80%	89.10%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	217.0	300.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4.8	0.5
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	217.0	300.0
Total Unrealized Capital Losses	4.8	0.5
U.S. corporate, state and municipalities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	2,929.4	479.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	159.3	6.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	236.0	22.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	29.1	0.9
More Than Twelve Months Below Amortized Cost, Fair Value	25.7	49.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	4.1	3.4
Total, Fair Value	3,191.1	551.7
Total Unrealized Capital Losses	192.5	11.1
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1,411.8	166.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	88.8	4.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	53.5	7.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	5.8	0.5
More Than Twelve Months Below Amortized Cost, Fair Value	44.2	87.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.1	11.2
Total, Fair Value	1,509.5	262.3
Total Unrealized Capital Losses	100.7	16.4
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	368.6	68.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	10.6	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	49.3	7.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.7	0.3
More Than Twelve Months Below Amortized Cost, Fair Value	85.2	132.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.7	16.2
Total, Fair Value	503.1	208.3
Total Unrealized Capital Losses	25.0	18.1
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	0	7.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.2	1.6
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	1.9	2.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.1	0.1
Total, Fair Value	2.1	11.6
Total Unrealized Capital Losses	0.1	0.2
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	62.4	15.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	4.0	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	27.7	34.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.7	6.7
Total, Fair Value	94.1	49.8
Total Unrealized Capital Losses	$ 3.9	$ 6.7