Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 944.7	$ 6,399.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	10.0	201.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	2,469.8	140.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	71.4	4.0
More Than Twelve Months Below Amortized Cost, Fair Value	435.8	571.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	56.0	73.8
Total, Fair Value	3,850.3	7,111.5
Total Unrealized Capital Losses	$ 137.4	$ 279.5
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	88.60%	88.60%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 51.8	$ 134.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	35.8	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	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	87.6	134.4
Total Unrealized Capital Losses	0.5	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	31.4	426.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	14.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	173.0	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	5.8	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	5.1	4.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.5	0.7
Total, Fair Value	209.5	431.5
Total Unrealized Capital Losses	6.5	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	259.3	2,107.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.2	53.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	549.6	2.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	14.9	0.2
More Than Twelve Months Below Amortized Cost, Fair Value	83.7	179.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12.0	18.6
Total, Fair Value	892.6	2,289.3
Total Unrealized Capital Losses	29.1	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	110.4	1,011.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.4	48.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	578.5	21.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	18.1	1.0
More Than Twelve Months Below Amortized Cost, Fair Value	119.8	131.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	26.4	30.5
Total, Fair Value	808.7	1,164.4
Total Unrealized Capital Losses	46.9	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	97.4	678.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.6	20.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	118.9	1.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.0	0.1
More Than Twelve Months Below Amortized Cost, Fair Value	91.4	132.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12.6	17.1
Total, Fair Value	307.7	812.1
Total Unrealized Capital Losses	16.2	37.6
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	108.6	599.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.9	22.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	224.5	0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	9.0	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	358.9	644.5
Total Unrealized Capital Losses	10.8	25.9
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	82.3	881.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.0	23.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	542.3	109.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	11.9	2.7
More Than Twelve Months Below Amortized Cost, Fair Value	93.7	54.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.4	2.8
Total, Fair Value	718.3	1,044.7
Total Unrealized Capital Losses	16.3	29.0
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	141.4	414.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.2	16.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	198.6	4.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	9.1	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	340.0	425.5
Total Unrealized Capital Losses	10.3	16.7
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	62.1	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	48.6	0.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	16.3	17.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.2	1.0
Total, Fair Value	127.0	165.1
Total Unrealized Capital Losses	$ 0.8	$ 1.3