Investments - Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 4,900.0	$ 1,038.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	277.1	13.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	479.1	39.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	56.0	1.7
More Than Twelve Months Below Amortized Cost, Fair Value	215.1	306.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	27.3	37.6
Total, Fair Value	5,594.2	1,383.7
Total Unrealized Capital Losses	360.4	53.0
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	88.70%	89.10%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	144.5	300.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.4	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	144.5	300.0
Total Unrealized Capital Losses	2.4	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	3,116.0	479.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	183.4	6.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	294.7	22.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	35.6	0.9
More Than Twelve Months Below Amortized Cost, Fair Value	40.9	49.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.9	3.4
Total, Fair Value	3,451.6	551.7
Total Unrealized Capital Losses	225.9	11.1
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1,280.2	166.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	82.3	4.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	158.5	7.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	19.4	0.5
More Than Twelve Months Below Amortized Cost, Fair Value	39.6	87.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.4	11.2
Total, Fair Value	1,478.3	262.3
Total Unrealized Capital Losses	107.1	16.4
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	299.3	68.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	8.8	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	19.7	7.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.0	0.3
More Than Twelve Months Below Amortized Cost, Fair Value	108.1	132.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.5	16.2
Total, Fair Value	427.1	208.3
Total Unrealized Capital Losses	21.3	18.1
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	3.8	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	0.1	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	0	2.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0.1
Total, Fair Value	3.9	11.6
Total Unrealized Capital Losses	0	0.2
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	56.2	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	6.1	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	26.5	34.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.5	6.7
Total, Fair Value	88.8	49.8
Total Unrealized Capital Losses	$ 3.7	$ 6.7