Investments - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 6,399.8	$ 3,909.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	201.7	160.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	140.2	3,089.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Losses	4.0	261.2
More Than Twelve Months Below Amortized Cost, Fair Value	571.5	602.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	73.8	132.9
Total Fair Value	7,111.5	7,602.4
Total Unrealized Capital Losses	$ 279.5	$ 554.2
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	88.60%	81.90%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 134.4	$ 69.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.7	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 Losses	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	134.4	69.4
Total Unrealized Capital Losses	1.7	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	426.6	191.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	14.9	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 Losses	0.0	5.7
More Than Twelve Months Below Amortized Cost, Fair Value	4.9	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.7	0.0
Total Fair Value	431.5	341.6
Total Unrealized Capital Losses	15.6	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	2,107.1	1,764.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	53.2	67.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	2.5	1,708.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Losses	0.2	136.4
More Than Twelve Months Below Amortized Cost, Fair Value	179.7	209.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	18.6	52.8
Total Fair Value	2,289.3	3,681.9
Total Unrealized Capital Losses	72.0	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	1,011.2	373.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	48.2	10.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	21.7	410.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Losses	1.0	43.8
More Than Twelve Months Below Amortized Cost, Fair Value	131.5	35.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	30.5	7.7
Total Fair Value	1,164.4	819.5
Total Unrealized Capital Losses	79.7	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	678.4	670.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	20.4	33.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1.2	485.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Losses	0.1	55.8
More Than Twelve Months Below Amortized Cost, Fair Value	132.5	195.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	17.1	61.9
Total Fair Value	812.1	1,351.5
Total Unrealized Capital Losses	37.6	151.5
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	599.7	546.0
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	22.8	42.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0.0	213.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Losses	0.0	16.5
More Than Twelve Months Below Amortized Cost, Fair Value	44.8	19.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.1	4.9
Total Fair Value	644.5	778.9
Total Unrealized Capital Losses	25.9	63.5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	881.2	116.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	23.5	1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	109.0	42.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Losses	2.7	0.9
More Than Twelve Months Below Amortized Cost, Fair Value	54.5	128.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.8	4.3
Total Fair Value	1,044.7	287.2
Total Unrealized Capital Losses	29.0	6.9
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	414.3	156.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	16.7	1.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	4.9	78.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Losses	0.0	2.1
More Than Twelve Months Below Amortized Cost, Fair Value	6.3	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total Fair Value	425.5	235.7
Total Unrealized Capital Losses	16.7	3.5
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	146.9	22.6
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	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 Losses	0.0	0.0
More Than Twelve Months Below Amortized Cost, Fair Value	17.3	13.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.0	1.3
Total Fair Value	165.1	36.7
Total Unrealized Capital Losses	$ 1.3	$ 1.4