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	$ 7.1		$ 34.7
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, Unrealized Capital Loss	44.6		50.9
Total Unrealized Capital Losses	181.8		368.8
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	853.5		2,054.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	18.8		45.3
Six months or less below amortized cost, Number of Securities	151		322
More than six months and twelve months or less below amortized cost, Amortized Cost	3,134.7		3,991.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	128.0		272.6
More than six months and twelve months or less below amortized cost, Number of Securities	398		502
More than twelve months below amortized cost, Amortized Cost	521.1		420.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	32.9		37.3
More than twelve months below amortized cost, Number of Securities	147		137
Total Amortized Cost	4,509.3		6,466.2
Total Unrealized Capital Losses	179.7		355.2
Total Number of Securities	696		961
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	4.1		24.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.0		5.3
Six months or less below amortized cost, Number of Securities	2		7
More than six months and twelve months or less below amortized cost, Amortized Cost	0		23.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0		5.8
More than six months and twelve months or less below amortized cost, Number of Securities	0		3
More than twelve months below amortized cost, Amortized Cost	4.2		9.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.1		2.5
More than twelve months below amortized cost, Number of Securities	4		8
Total Amortized Cost	8.3		57.1
Total Unrealized Capital Losses	2.1		13.6
Total Number of Securities	6		18
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
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, Unrealized Capital Loss	0.7		0.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0		0
Total Unrealized Capital Losses	1.9		2.9
U.S. Treasuries | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	161.6		161.5
Total Unrealized Capital Losses	1.9		2.9
Total Number of Securities	4		4
U.S. Treasuries | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0		0
Total Unrealized Capital Losses	0		0
Total Number of Securities	0		0
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]
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0
Total Unrealized Capital Losses	0	[1]
U.S. government agencies and authorities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	2.4
Total Unrealized Capital Losses	0	[1]
Total Number of Securities	1
U.S. government agencies and authorities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0
Total Unrealized Capital Losses	0
Total Number of Securities	0
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	3.5		22.9
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, Unrealized Capital Loss	22.7		32.2
Total Unrealized Capital Losses	107.3		238.9
U.S. corporate, state and municipalities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	2,673.9		3,869.0
Total Unrealized Capital Losses	106.3		233.2
Total Number of Securities	349		519
U.S. corporate, state and municipalities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	4.0		22.8
Total Unrealized Capital Losses	1.0		5.7
Total Number of Securities	2		2
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
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, Unrealized Capital Loss	39.6		86.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	14.3		7.9
Total Unrealized Capital Losses	55.1		100.0
Foreign | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,107.7		1,665.8
Total Unrealized Capital Losses	55.1		95.0
Total Number of Securities	162		239
Foreign | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0		23.1
Total Unrealized Capital Losses	0		5.0
Total Number of Securities	0		5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
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, Unrealized Capital Loss	8.6		12.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.0		7.8
Total Unrealized Capital Losses	14.5		22.7
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	467.2		596.9
Total Unrealized Capital Losses	14.3		21.0
Total Number of Securities	154		162
Residential mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0.7		6.8
Total Unrealized Capital Losses	0.2		1.7
Total Number of Securities	1		7
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
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, Unrealized Capital Loss	0		0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0		0
Total Unrealized Capital Losses	0.1		0.9
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	4.9		78.8
Total Unrealized Capital Losses	0.1		0.9
Total Number of Securities	3		12
Commercial mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0		0
Total Unrealized Capital Losses	0		0
Total Number of Securities	0		0
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
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, Unrealized Capital Loss	0.1		0.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.6		3.0
Total Unrealized Capital Losses	2.9		3.4
Other asset-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	91.6		94.2
Total Unrealized Capital Losses	2.0		2.2
Total Number of Securities	23		25
Other asset-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	3.6		4.4
Total Unrealized Capital Losses	$ 0.9		$ 1.2
Total Number of Securities	3		4

[1] Less than $0.1.