Investments - Unrealized Capital Losses 1 (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 1.5		$ 34.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.5		283.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	75.1		50.9
Total Unrealized Capital Losses	80.1		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	422.3		2,054.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	6.1		45.3
Six months or less below amortized cost, Number of Securities	90		322
More than six months and twelve months or less below amortized cost, Amortized Cost	94.3		3,991.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	4.5		272.6
More than six months and twelve months or less below amortized cost, Number of Securities	30		502
More than twelve months below amortized cost, Amortized Cost	2,199.0		420.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	68.9		37.3
More than twelve months below amortized cost, Number of Securities	344		137
Total Amortized Cost	2,715.6		6,466.2
Total Unrealized Capital Losses	79.5		355.2
Total Number of Securities	464		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	1		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	2.2		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	2.2		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	0.1		0
Total Unrealized Capital Losses	0.2		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	67.1		161.5
Total Unrealized Capital Losses	0.2		2.9
Total Number of Securities	3		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	2.5
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	0.5		22.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.2		183.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	42.2		32.2
Total Unrealized Capital Losses	45.9		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	1,536.9		3,869.0
Total Unrealized Capital Losses	45.9		233.2
Total Number of Securities	210		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	0.2		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	22.7		7.9
Total Unrealized Capital Losses	23.0		100.0
Foreign | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	709.4		1,665.8
Total Unrealized Capital Losses	23.0		95.0
Total Number of Securities	97		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	0.5		2.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2		12.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	8.1		7.8
Total Unrealized Capital Losses	8.8		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	328.5		596.9
Total Unrealized Capital Losses	8.8		21.0
Total Number of Securities	133		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	0.2		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	0.2		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	27.9		78.8
Total Unrealized Capital Losses	0.2		0.9
Total Number of Securities	4		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		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	2.0		3.0
Total Unrealized Capital Losses	2.0		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	43.3		94.2
Total Unrealized Capital Losses	1.4		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	2.2		4.4
Total Unrealized Capital Losses	$ 0.6		$ 1.2
Total Number of Securities	2		4

[1] Less than $0.1.