Investments - Unrealized Capital Losses (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 761.5		$ 1,955.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	7.1		34.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	3,011.1		3,754.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	130.1		283.2
More Than Twelve Months Below Amortized Cost, Fair Value	563.2		444.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	44.6		50.9
Total, Fair Value	4,335.8		6,154.5
Total Unrealized Capital Losses	181.8		368.8
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	92.70%		89.70%
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	125.4		124.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.2		2.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	34.3		34.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.7		0.8
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	159.7		158.6
Total Unrealized Capital Losses	1.9		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	2.4
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	2.4
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	319.5		1,002.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	3.5		22.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	1,969.3		2,413.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	81.1		183.8
More Than Twelve Months Below Amortized Cost, Fair Value	281.8		236.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	22.7		32.2
Total, Fair Value	2,570.6		3,652.9
Total Unrealized Capital Losses	107.3		238.9
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	115.0		448.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.2		5.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	780.1		1,063.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	39.6		86.4
More Than Twelve Months Below Amortized Cost, Fair Value	157.5		76.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	14.3		7.9
Total, Fair Value	1,052.6		1,588.9
Total Unrealized Capital Losses	55.1		100.0
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	157.6		262.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.9		2.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	197.0		212.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	8.6		12.0
More Than Twelve Months Below Amortized Cost, Fair Value	98.8		105.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.0		7.8
Total, Fair Value	453.4		581.0
Total Unrealized Capital Losses	14.5		22.7
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	4.8		77.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1		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	4.8		77.9
Total Unrealized Capital Losses	0.1		0.9
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	36.8		38.9
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2		0.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	30.4		30.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1		0.2
More Than Twelve Months Below Amortized Cost, Fair Value	25.1		26.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.6		3.0
Total, Fair Value	92.3		95.2
Total Unrealized Capital Losses	$ 2.9		$ 3.4

[1] Less than $0.1.