Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 4,430.8	$ 1,660.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	216.1	56.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	652.1	69.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	67.2	2.3
More Than Twelve Months Below Amortized Cost, Fair Value	543.8	1,244.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	85.4	53.8
Total, Fair Value	5,626.7	2,975.1
Total Unrealized Capital Losses	$ 368.7	$ 112.8
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	86.40%	95.90%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 0.0	$ 12.4
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	12.4
Total Unrealized Capital Losses	0.0	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	212.0	22.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	5.3	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	12.2	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.8	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	224.2	22.5
Total Unrealized Capital Losses	6.1	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	2,235.9	611.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	101.1	18.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	315.7	14.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	32.2	1.4
More Than Twelve Months Below Amortized Cost, Fair Value	202.2	612.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	34.8	20.8
Total, Fair Value	2,753.8	1,239.5
Total Unrealized Capital Losses	168.1	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	501.1	160.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	30.7	2.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	40.4	19.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.2	0.1
More Than Twelve Months Below Amortized Cost, Fair Value	36.4	100.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	7.1	6.8
Total, Fair Value	577.9	280.2
Total Unrealized Capital Losses	41.0	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	728.4	545.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	55.6	33.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	232.3	9.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	28.3	0.2
More Than Twelve Months Below Amortized Cost, Fair Value	146.5	324.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	36.7	16.6
Total, Fair Value	1,107.2	879.5
Total Unrealized Capital Losses	120.6	50.3
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	586.5	125.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	21.6	2.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	19.0	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.5	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	18.9	25.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.0	3.5
Total, Fair Value	624.4	151.4
Total Unrealized Capital Losses	26.1	5.7
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	50.5	94.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.7	0.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	27.5	25.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	0.6
More Than Twelve Months Below Amortized Cost, Fair Value	125.8	163.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.4	4.5
Total, Fair Value	203.8	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	103.2	59.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.1	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	4.9	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	108.1	59.1
Total Unrealized Capital Losses	1.1	0.1
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	13.2	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	14.0	18.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.4	1.6
Total, Fair Value	27.3	45.4
Total Unrealized Capital Losses	$ 1.4	$ 1.7