Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 4,443.5	$ 6,399.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	113.6	201.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	690.5	140.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	40.5	4.0
More Than Twelve Months Below Amortized Cost, Fair Value	467.7	571.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	54.3	73.8
Total, Fair Value	5,601.7	7,111.5
Total Unrealized Capital Losses	$ 208.4	$ 279.5
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	89.60%	88.60%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 131.0	$ 134.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	10.3	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	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	141.3	134.4
Total Unrealized Capital Losses	0.4	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	297.4	426.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	8.9	14.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	45.9	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.3	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	4.8	4.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.7	0.7
Total, Fair Value	348.1	431.5
Total Unrealized Capital Losses	12.9	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	1,360.9	2,107.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	30.2	53.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	142.3	2.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	9.0	0.2
More Than Twelve Months Below Amortized Cost, Fair Value	120.0	179.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12.6	18.6
Total, Fair Value	1,623.2	2,289.3
Total Unrealized Capital Losses	51.8	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	757.5	1,011.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	27.2	48.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	83.3	21.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	9.0	1.0
More Than Twelve Months Below Amortized Cost, Fair Value	109.7	131.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	25.4	30.5
Total, Fair Value	950.5	1,164.4
Total Unrealized Capital Losses	61.6	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	433.6	678.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	9.2	20.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	12.6	1.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5	0.1
More Than Twelve Months Below Amortized Cost, Fair Value	118.8	132.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.2	17.1
Total, Fair Value	565.0	812.1
Total Unrealized Capital Losses	20.9	37.6
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	360.7	599.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	9.1	22.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	76.3	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	6.9	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	25.8	44.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.9	3.1
Total, Fair Value	462.8	644.5
Total Unrealized Capital Losses	16.9	25.9
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	613.6	881.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	14.6	23.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	287.5	109.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	9.7	2.7
More Than Twelve Months Below Amortized Cost, Fair Value	71.1	54.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.2	2.8
Total, Fair Value	972.2	1,044.7
Total Unrealized Capital Losses	27.5	29.0
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	426.5	414.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	13.7	16.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	22.3	4.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.0	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	0.8	6.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total, Fair Value	449.6	425.5
Total Unrealized Capital Losses	15.7	16.7
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	62.3	146.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.4	0.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	10.0	0.9
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	16.7	17.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.3	1.0
Total, Fair Value	89.0	165.1
Total Unrealized Capital Losses	$ 0.7	$ 1.3