Investments - Unrealized Capital Losses 1 (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, Unrealized Capital Loss	$ 28.5		$ 131.3
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, Unrealized Capital Loss	186.7		205.2
Total Unrealized Capital Losses	603.9		1,136.5
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
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, Unrealized Capital Losses	29.4		52.6
More than twelve months below amortized cost, Unrealized Capital Loss	6.8		9.9
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, Unrealized Capital Loss	0	[1]	0	[1]
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, Unrealized Capital Loss	0		0
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, Unrealized Capital Loss	14.2		78.5
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, Unrealized Capital Loss	100.7		109.6
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, Unrealized Capital Loss	2.4		16.0
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, Unrealized Capital Loss	25.9		18.4
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, Unrealized Capital Loss	2.7		8.3
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, Unrealized Capital Loss	23.0		31.5
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, Unrealized Capital Loss	0.6		3.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0		0
More than twelve months below amortized cost, Unrealized Capital Loss	0		0.2
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, Unrealized Capital Loss	0.7		0.9
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, Unrealized Capital Loss	30.3		35.6
Total Unrealized Capital Losses	31.4		38.0
Fair value decline below amortized cost less than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Amortized Cost	3,892.3		7,883.3
Six months or less below amortized cost, Unrealized Capital Loss	69.1		166.0
Six months or less below amortized cost, Number of Securities	322		570
More than six months and twelve months or less below amortized cost, Amortized Cost	9,935.5		12,339.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	381.6		776.8
More than six months and twelve months or less below amortized cost, Number of Securities	622		798
More than twelve months below amortized cost, Amortized Cost	2,004.7		1,579.2
More than twelve months below amortized cost, Unrealized Capital Loss	136.9		144.5
More than twelve months below amortized cost, Number of Securities	338		302
Total Amortized Cost	15,832.5		21,802.2
Total Unrealized Capital Losses	587.6		1,087.3
Number of Securities	1,282		1,670
Fair value decline below amortized cost less than 20% | U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	2,332.8		2,750.5
Total Unrealized Capital Losses	44.1		81.4
Number of Securities	28		32
Fair value decline below amortized cost less than 20% | U.S. government agencies and authorities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	64.5		66.8
Total Unrealized Capital Losses	0.2		1.4
Number of Securities	3		3
Fair value decline below amortized cost less than 20% | U.S. corporate, state and municipalities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	8,506.0		11,892.6
Total Unrealized Capital Losses	355.1		694.2
Number of Securities	520		744
Fair value decline below amortized cost less than 20% | Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	2,593.3		3,944.2
Total Unrealized Capital Losses	112.0		211.0
Number of Securities	210		300
Fair value decline below amortized cost less than 20% | Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	1,814.6		2,361.4
Total Unrealized Capital Losses	49.0		70.2
Number of Securities	416		471
Fair value decline below amortized cost less than 20% | Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	29.2		245.5
Total Unrealized Capital Losses	0.6		3.5
Number of Securities	8		16
Fair value decline below amortized cost less than 20% | Other asset-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	492.1		541.2
Total Unrealized Capital Losses	26.6		25.6
Number of Securities	97		104
Fair value decline below amortized cost greater than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Amortized Cost	35.4		80.5
Six months or less below amortized cost, Unrealized Capital Loss	8.5		18.6
Six months or less below amortized cost, Number of Securities	9		20
More than six months and twelve months or less below amortized cost, Amortized Cost	12.9		67.6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	4.1		16.7
More than six months and twelve months or less below amortized cost, Number of Securities	5		8
More than twelve months below amortized cost, Amortized Cost	11.6		55.8
More than twelve months below amortized cost, Unrealized Capital Loss	3.7		13.9
More than twelve months below amortized cost, Number of Securities	15		22
Total Amortized Cost	59.9		203.9
Total Unrealized Capital Losses	16.3		49.2
Number of Securities	29		50
Fair value decline below amortized cost greater than 20% | U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	0		25.3
Total Unrealized Capital Losses	0		5.4
Number of Securities	0		1
Fair value decline below amortized cost greater than 20% | U.S. government agencies and authorities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	0		0
Total Unrealized Capital Losses	0		0
Number of Securities	0		0
Fair value decline below amortized cost greater than 20% | U.S. corporate, state and municipalities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	28.6		55.8
Total Unrealized Capital Losses	6.9		13.5
Number of Securities	3		5
Fair value decline below amortized cost greater than 20% | Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	3.4		35.0
Total Unrealized Capital Losses	0.8		7.7
Number of Securities	1		6
Fair value decline below amortized cost greater than 20% | Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	7.9		35.6
Total Unrealized Capital Losses	3.8		10.2
Number of Securities	15		25
Fair value decline below amortized cost greater than 20% | Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	0	[1]	0
Total Unrealized Capital Losses	0	[1]	0
Number of Securities	1		0
Fair value decline below amortized cost greater than 20% | Other asset-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	20.0		52.2
Total Unrealized Capital Losses	$ 4.8		$ 12.4
Number of Securities	9		13

[1] * Less than $0.1