Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 357.4	$ 3,909.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4.1	160.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	252.2	3,089.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	12.2	261.2
More Than Twelve Months Below Amortized Cost, Fair Value	994.0	602.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	124.3	132.9
Total, Fair Value	1,603.6	7,602.4
Total Unrealized Capital Losses	$ 140.6	$ 554.2
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	88.90%	81.90%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 0.0	$ 69.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	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	0.0	69.4
Total Unrealized Capital Losses	0.0	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	1.1	191.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	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	5.3	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.3	0.0
Total, Fair Value	6.4	341.6
Total Unrealized Capital Losses	0.3	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	75.7	1,764.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.4	67.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	108.6	1,708.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	4.2	136.4
More Than Twelve Months Below Amortized Cost, Fair Value	440.3	209.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	34.4	52.8
Total, Fair Value	624.6	3,681.9
Total Unrealized Capital Losses	40.0	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	26.4	373.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	10.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	14.1	410.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5	43.8
More Than Twelve Months Below Amortized Cost, Fair Value	132.4	35.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	30.7	7.7
Total, Fair Value	172.9	819.5
Total Unrealized Capital Losses	31.5	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	13.0	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	46.6	485.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.7	55.8
More Than Twelve Months Below Amortized Cost, Fair Value	299.5	195.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	51.6	61.9
Total, Fair Value	359.1	1,351.5
Total Unrealized Capital Losses	54.9	151.5
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	0.4	546.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	42.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	37.9	213.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.4	16.5
More Than Twelve Months Below Amortized Cost, Fair Value	50.6	19.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.2	4.9
Total, Fair Value	88.9	778.9
Total Unrealized Capital Losses	6.6	63.5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	226.4	116.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.8	1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	16.5	42.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.3	0.9
More Than Twelve Months Below Amortized Cost, Fair Value	53.0	128.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.8	4.3
Total, Fair Value	295.9	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	13.1	156.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	1.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	17.3	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	30.4	235.7
Total Unrealized Capital Losses	0.1	3.5
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1.3	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	11.2	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	12.9	13.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.3	1.3
Total, Fair Value	25.4	36.7
Total Unrealized Capital Losses	$ 1.3	$ 1.4