Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 1,661.7	$ 6,399.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	17.7	201.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1,199.6	140.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	35.4	4.0
More Than Twelve Months Below Amortized Cost, Fair Value	802.1	571.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	60.8	73.8
Total, Fair Value	3,663.4	7,111.5
Total Unrealized Capital Losses	$ 113.9	$ 279.5
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	93.00%	88.60%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 25.6	$ 134.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	31.2	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	9.4	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.1	0.0
Total, Fair Value	66.2	134.4
Total Unrealized Capital Losses	0.3	1.7
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	64.4	426.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.6	14.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	70.6	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.4	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	39.2	4.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.9	0.7
Total, Fair Value	174.2	431.5
Total Unrealized Capital Losses	4.9	15.6
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	380.3	2,107.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4.0	53.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	176.7	2.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	6.5	0.2
More Than Twelve Months Below Amortized Cost, Fair Value	184.2	179.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12.0	18.6
Total, Fair Value	741.2	2,289.3
Total Unrealized Capital Losses	22.5	72.0
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	231.4	1,011.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.0	48.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	373.9	21.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	12.0	1.0
More Than Twelve Months Below Amortized Cost, Fair Value	164.1	131.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	26.3	30.5
Total, Fair Value	769.4	1,164.4
Total Unrealized Capital Losses	40.3	79.7
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	88.5	678.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.6	20.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	31.3	1.2
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	80.0	132.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	8.6	17.1
Total, Fair Value	199.8	812.1
Total Unrealized Capital Losses	10.8	37.6
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	182.8	599.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	5.1	22.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	87.1	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.7	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	98.6	44.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.2	3.1
Total, Fair Value	368.5	644.5
Total Unrealized Capital Losses	13.0	25.9
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	232.3	881.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.8	23.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	327.8	109.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	6.4	2.7
More Than Twelve Months Below Amortized Cost, Fair Value	205.7	54.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.6	2.8
Total, Fair Value	765.8	1,044.7
Total Unrealized Capital Losses	14.8	29.0
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	361.4	414.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	3.3	16.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	78.6	4.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.4	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	0.0	6.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total, Fair Value	440.0	425.5
Total Unrealized Capital Losses	6.7	16.7
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	95.0	146.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	0.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	22.4	0.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	20.9	17.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.1	1.0
Total, Fair Value	138.3	165.1
Total Unrealized Capital Losses	$ 0.6	$ 1.3