Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 930.7	$ 3,909.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	9.6	160.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	105.3	3,089.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.6	261.2
More Than Twelve Months Below Amortized Cost, Fair Value	667.2	602.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	86.3	132.9
Total, Fair Value	1,703.2	7,602.4
Total Unrealized Capital Losses	$ 99.5	$ 554.2
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	88.50%	81.90%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 30.1	$ 69.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	0.3
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	30.1	69.4
Total Unrealized Capital Losses	0.1	0.3
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	43.6	191.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.8	2.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.0	150.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	5.7
More Than Twelve Months Below Amortized Cost, Fair Value	5.2	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.3	0.0
Total, Fair Value	48.8	341.6
Total Unrealized Capital Losses	1.1	7.9
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	247.9	1,764.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.8	67.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	25.6	1,708.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	136.4
More Than Twelve Months Below Amortized Cost, Fair Value	207.5	209.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	21.2	52.8
Total, Fair Value	481.0	3,681.9
Total Unrealized Capital Losses	24.3	256.8
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	101.9	373.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.2	10.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.0	410.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	43.8
More Than Twelve Months Below Amortized Cost, Fair Value	139.2	35.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	24.1	7.7
Total, Fair Value	241.1	819.5
Total Unrealized Capital Losses	26.3	62.4
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	45.5	670.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.6	33.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	5.1	485.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.3	55.8
More Than Twelve Months Below Amortized Cost, Fair Value	187.3	195.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	32.9	61.9
Total, Fair Value	237.9	1,351.5
Total Unrealized Capital Losses	34.8	151.5
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	81.8	546.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.8	42.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	10.1	213.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	16.5
More Than Twelve Months Below Amortized Cost, Fair Value	51.5	19.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.6	4.9
Total, Fair Value	143.4	778.9
Total Unrealized Capital Losses	5.7	63.5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	345.9	116.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.2	1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	41.6	42.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.6	0.9
More Than Twelve Months Below Amortized Cost, Fair Value	54.2	128.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.1	4.3
Total, Fair Value	441.7	287.2
Total Unrealized Capital Losses	5.9	6.9
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	23.9	156.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	1.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	22.0	78.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	2.1
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	45.9	235.7
Total Unrealized Capital Losses	0.2	3.5
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	10.1	22.6
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	0.9	0.4
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	22.3	13.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.1	1.3
Total, Fair Value	33.3	36.7
Total Unrealized Capital Losses	$ 1.1	$ 1.4