Investments - Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 359.9		$ 1,955.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.5		34.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	72.3		3,754.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.5		283.2
More Than Twelve Months Below Amortized Cost, Fair Value	2,205.5		444.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	75.1		50.9
Total, Fair Value	2,637.7		6,154.5
Total Unrealized Capital Losses	80.1		368.8
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	96.70%		89.70%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	62.0		124.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1		2.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0		34.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0		0.8
More Than Twelve Months Below Amortized Cost, Fair Value	4.9		0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.1		0
Total, Fair Value	66.9		158.6
Total Unrealized Capital Losses	0.2		2.9
U.S. government agencies and authorities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	2.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	[1]
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0
More Than Twelve Months Below Amortized Cost, Fair Value	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0
Total, Fair Value	2.5
Total Unrealized Capital Losses	0	[1]
U.S. corporate, state and municipalities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	144.5		1,002.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5		22.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	47.8		2,413.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.2		183.8
More Than Twelve Months Below Amortized Cost, Fair Value	1,298.7		236.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	42.2		32.2
Total, Fair Value	1,491.0		3,652.9
Total Unrealized Capital Losses	45.9		238.9
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	54.3		448.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2		5.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	13.3		1,063.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1		86.4
More Than Twelve Months Below Amortized Cost, Fair Value	618.8		76.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	22.7		7.9
Total, Fair Value	686.4		1,588.9
Total Unrealized Capital Losses	23.0		100.0
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	67.2		262.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5		2.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	11.2		212.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2		12.0
More Than Twelve Months Below Amortized Cost, Fair Value	241.3		105.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	8.1		7.8
Total, Fair Value	319.7		581.0
Total Unrealized Capital Losses	8.8		22.7
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	27.7		77.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2		0.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0		0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0		0
More Than Twelve Months Below Amortized Cost, Fair Value	0		0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0		0
Total, Fair Value	27.7		77.9
Total Unrealized Capital Losses	0.2		0.9
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1.7		38.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0		0.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0		30.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0		0.2
More Than Twelve Months Below Amortized Cost, Fair Value	41.8		26.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.0		3.0
Total, Fair Value	43.5		95.2
Total Unrealized Capital Losses	$ 2.0		$ 3.4

[1] Less than $0.1.