Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 4,822.6	$ 1,660.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	162.8	56.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	371.6	69.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	44.1	2.3
More Than Twelve Months Below Amortized Cost, Fair Value	425.9	1,244.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	37.5	53.8
Total, Fair Value	5,620.1	2,975.1
Total Unrealized Capital Losses	$ 244.4	$ 112.8
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	91.90%	95.90%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 15.8	$ 12.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.0	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total, Fair Value	15.8	12.4
Total Unrealized Capital Losses	0.1	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	0.0	2.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.0	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total, Fair Value	0.0	2.3
Total Unrealized Capital Losses	0.0	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	218.9	22.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	8.5	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.0	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total, Fair Value	218.9	22.5
Total Unrealized Capital Losses	8.5	0.1
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	2,521.8	611.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	86.4	18.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	149.6	14.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	13.3	1.4
More Than Twelve Months Below Amortized Cost, Fair Value	143.8	612.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.8	20.8
Total, Fair Value	2,815.2	1,239.5
Total Unrealized Capital Losses	111.5	40.2
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	579.1	160.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	27.0	2.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	17.4	19.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.6	0.1
More Than Twelve Months Below Amortized Cost, Fair Value	18.9	100.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.6	6.8
Total, Fair Value	615.4	280.2
Total Unrealized Capital Losses	34.2	8.9
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	812.4	545.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	24.6	33.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	178.9	9.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	27.3	0.2
More Than Twelve Months Below Amortized Cost, Fair Value	103.3	324.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12.3	16.6
Total, Fair Value	1,094.6	879.5
Total Unrealized Capital Losses	64.2	50.3
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	344.4	125.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	9.0	2.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	13.7	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.3	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	9.6	25.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.6	3.5
Total, Fair Value	367.7	151.4
Total Unrealized Capital Losses	11.9	5.7
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	80.3	94.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.1	0.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	11.9	25.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.6	0.6
More Than Twelve Months Below Amortized Cost, Fair Value	132.5	163.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	4.8	4.5
Total, Fair Value	224.7	282.8
Total Unrealized Capital Losses	6.5	5.8
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	248.7	59.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	6.1	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.0	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total, Fair Value	248.7	59.1
Total Unrealized Capital Losses	6.1	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	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.1	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	17.8	18.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.4	1.6
Total, Fair Value	19.1	45.4
Total Unrealized Capital Losses	$ 1.4	$ 1.7