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	$ 3,586.4		$ 7,533.0
Six months or less below amortized cost, Unrealized Capital Loss	28.5		131.3
More than six months and twelve months or less below amortized cost, Fair Value	9,568.0		11,610.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	388.7		800.0
More than twelve months below amortized cost, Fair Value	2,134.1		1,726.3
More than twelve months below amortized cost, Unrealized Capital Loss	186.7		205.2
Total, Fair Value	15,288.5		20,869.6
Total Unrealized Capital Losses	603.9		1,136.5
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	92.00%		89.40%
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	1,315.7		1,559.5
Six months or less below amortized cost, Unrealized Capital Loss	7.9		24.3
More than six months and twelve months or less below amortized cost, Fair Value	927.9		1,087.6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	29.4		52.6
More than twelve months below amortized cost, Fair Value	45.1		41.9
More than twelve months below amortized cost, Unrealized Capital Loss	6.8		9.9
Total, Fair Value	2,288.7		2,689.0
Total Unrealized Capital Losses	44.1		86.8
U.S. government agencies and authorities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	6.8		9.5
Six months or less below amortized cost, Unrealized Capital Loss	0	[1]	0	[1]
More than six months and twelve months or less below amortized cost, Fair Value	57.5		55.9
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.2		1.4
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	64.3		65.4
Total Unrealized Capital Losses	0.2		1.4
U.S. corporate, state and municipalities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	1,316.4		3,524.9
Six months or less below amortized cost, Unrealized Capital Loss	14.2		78.5
More than six months and twelve months or less below amortized cost, Fair Value	5,743.4		6,893.9
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	247.1		519.6
More than twelve months below amortized cost, Fair Value	1,112.8		821.9
More than twelve months below amortized cost, Unrealized Capital Loss	100.7		109.6
Total, Fair Value	8,172.6		11,240.7
Total Unrealized Capital Losses	362.0		707.7
Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	345.1		1,133.6
Six months or less below amortized cost, Unrealized Capital Loss	2.4		16.0
More than six months and twelve months or less below amortized cost, Fair Value	1,842.9		2,447.8
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	84.5		184.3
More than twelve months below amortized cost, Fair Value	295.9		179.1
More than twelve months below amortized cost, Unrealized Capital Loss	25.9		18.4
Total, Fair Value	2,483.9		3,760.5
Total Unrealized Capital Losses	112.8		218.7
Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	427.1		919.1
Six months or less below amortized cost, Unrealized Capital Loss	2.7		8.3
More than six months and twelve months or less below amortized cost, Fair Value	950.4		1,019.6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	27.1		40.6
More than twelve months below amortized cost, Fair Value	392.2		377.9
More than twelve months below amortized cost, Unrealized Capital Loss	23.0		31.5
Total, Fair Value	1,769.7		2,316.6
Total Unrealized Capital Losses	52.8		80.4
Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	28.6		235.8
Six months or less below amortized cost, Unrealized Capital Loss	0.6		3.3
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 Losses	0		0
More than twelve months below amortized cost, Fair Value	0		6.2
More than twelve months below amortized cost, Unrealized Capital Loss	0		0.2
Total, Fair Value	28.6		242.0
Total Unrealized Capital Losses	0.6		3.5
Other asset-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	146.7		150.6
Six months or less below amortized cost, Unrealized Capital Loss	0.7		0.9
More than six months and twelve months or less below amortized cost, Fair Value	45.9		105.5
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.4		1.5
More than twelve months below amortized cost, Fair Value	288.1		299.3
More than twelve months below amortized cost, Unrealized Capital Loss	30.3		35.6
Total, Fair Value	480.7		555.4
Total Unrealized Capital Losses	$ 31.4		$ 38.0

[1] * Less than $0.1