Investments - Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 973.2	$ 1,660.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	28.7	56.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	220.3	69.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	23.6	2.3
More Than Twelve Months Below Amortized Cost, Fair Value	472.3	1,244.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	27.7	53.8
Total, Fair Value	1,665.8	2,975.1
Total Unrealized Capital Losses	80.0	112.8
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	94.50%	95.90%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	0	12.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	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	0	12.4
Total Unrealized Capital Losses	0	0
U.S. government agencies and authorities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	0	2.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	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	0	2.3
Total Unrealized Capital Losses	0	0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	13.5	22.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	0.1
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	13.5	22.5
Total Unrealized Capital Losses	0.1	0.1
U.S. corporate securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	475.7	772.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	12.7	20.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	109.7	34.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	8.5	1.5
More Than Twelve Months Below Amortized Cost, Fair Value	190.4	712.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	13.6	27.6
Total, Fair Value	775.8	1,519.7
Total Unrealized Capital Losses	34.8	49.1
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	414.2	671.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	14.9	35.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	109.2	9.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	15.1	0.2
More Than Twelve Months Below Amortized Cost, Fair Value	124.2	350.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	9.2	20.1
Total, Fair Value	647.6	1,030.9
Total Unrealized Capital Losses	39.2	56.0
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	55.4	94.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.9	0.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1.4	25.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0.6
More Than Twelve Months Below Amortized Cost, Fair Value	139.5	163.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.4	4.5
Total, Fair Value	196.3	282.8
Total Unrealized Capital Losses	4.3	5.8
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	13.2	59.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	0.1
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	13.2	59.1
Total Unrealized Capital Losses	0.1	0.1
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1.2	27.0
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	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	18.2	18.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.5	1.6
Total, Fair Value	19.4	45.4
Total Unrealized Capital Losses	$ 1.5	$ 1.7