Investments - Unrealized Capital Losses 1 (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 26.4		$ 34.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.8		283.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	78.8		50.9
Total Unrealized Capital Losses	107.0		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	1,725.6		2,054.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	26.5		45.3
Six months or less below amortized cost, Number of Securities	320		322
More than six months and twelve months or less below amortized cost, Amortized Cost	134.4		3,991.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	6.3		272.6
More than six months and twelve months or less below amortized cost, Number of Securities	36		502
More than twelve months below amortized cost, Amortized Cost	1,631.5		420.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	73.6		37.3
More than twelve months below amortized cost, Number of Securities	273		137
Total Amortized Cost	3,491.5		6,466.2
Total Unrealized Capital Losses	106.4		355.2
Total Number of Securities	629		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	0		24.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0		5.3
Six months or less below amortized cost, Number of Securities	0		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	1		3
More than twelve months below amortized cost, Amortized Cost	2.3		9.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.6		2.5
More than twelve months below amortized cost, Number of Securities	2		8
Total Amortized Cost	2.3		57.1
Total Unrealized Capital Losses	0.6		13.6
Total Number of Securities	3		18
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
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, Unrealized Capital Loss	0		0.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0		0
Total Unrealized Capital Losses	0.1		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	27.2		161.5
Total Unrealized Capital Losses	0.1		2.9
Total Number of Securities	2		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]	0	[1]
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	[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	2.5		0
Total Unrealized Capital Losses	0	[1]	0	[1]
Total Number of Securities	1		0
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		0
Total Unrealized Capital Losses	0		0
Total Number of Securities	0		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	15.9		22.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.4		183.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	45.0		32.2
Total Unrealized Capital Losses	62.3		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	2,034.9		3,869.0
Total Unrealized Capital Losses	62.3		233.2
Total Number of Securities	335		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	0		22.8
Total Unrealized Capital Losses	0		5.7
Total Number of Securities	1		2
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	9.4		5.7
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, Unrealized Capital Loss	25.9		7.9
Total Unrealized Capital Losses	35.4		100.0
Foreign | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	994.9		1,665.8
Total Unrealized Capital Losses	35.4		95.0
Total Number of Securities	148		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	0.4		2.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3		12.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.3		7.8
Total Unrealized Capital Losses	7.0		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	306.7		596.9
Total Unrealized Capital Losses	7.0		21.0
Total Number of Securities	122		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		6.8
Total Unrealized Capital Losses	0		1.7
Total Number of Securities	0		7
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.4		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	0.4		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	48.2		78.8
Total Unrealized Capital Losses	0.4		0.9
Total Number of Securities	5		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		0.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.6		3.0
Total Unrealized Capital Losses	1.8		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	77.1		94.2
Total Unrealized Capital Losses	1.2		2.2
Total Number of Securities	16		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	2.3		4.4
Total Unrealized Capital Losses	$ 0.6		$ 1.2
Total Number of Securities	2		4

[1] Less than $0.1.