Investments - Unrealized Capital Losses (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, Fair Value	$ 6,461.1	$ 7,533.0
Six months or less below amortized cost, Unrealized Capital Loss	83.5	131.3
More than six months and twelve months or less below amortized cost, Fair Value	162.8	11,610.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	5.4	800.0
More than twelve months below amortized cost, Fair Value	5,531.0	1,726.3
More than twelve months below amortized cost, Unrealized Capital Loss	260.2	205.2
Total, Fair Value	12,154.9	20,869.6
Total Unrealized Capital Losses	349.1	1,136.5
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	95.50%	89.40%
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	375.6	1,559.5
Six months or less below amortized cost, Unrealized Capital Loss	1.9	24.3
More than six months and twelve months or less below amortized cost, Fair Value	0	1,087.6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0	52.6
More than twelve months below amortized cost, Fair Value	70.3	41.9
More than twelve months below amortized cost, Unrealized Capital Loss	3.2	9.9
Total, Fair Value	445.9	2,689.0
Total Unrealized Capital Losses	5.1	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	6.9	9.5
Six months or less below amortized cost, Unrealized Capital Loss	0.1	0
More than six months and twelve months or less below amortized cost, Fair Value	0	55.9
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0	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	6.9	65.4
Total Unrealized Capital Losses	0.1	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	3,902.8	3,524.9
Six months or less below amortized cost, Unrealized Capital Loss	54.3	78.5
More than six months and twelve months or less below amortized cost, Fair Value	75.3	6,893.9
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	3.9	519.6
More than twelve months below amortized cost, Fair Value	3,536.4	821.9
More than twelve months below amortized cost, Unrealized Capital Loss	162.9	109.6
Total, Fair Value	7,514.5	11,240.7
Total Unrealized Capital Losses	221.1	707.7
Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	1,558.8	1,133.6
Six months or less below amortized cost, Unrealized Capital Loss	24.6	16.0
More than six months and twelve months or less below amortized cost, Fair Value	19.3	2,447.8
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.3	184.3
More than twelve months below amortized cost, Fair Value	916.5	179.1
More than twelve months below amortized cost, Unrealized Capital Loss	47.7	18.4
Total, Fair Value	2,494.6	3,760.5
Total Unrealized Capital Losses	72.6	218.7
Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	266.2	919.1
Six months or less below amortized cost, Unrealized Capital Loss	1.3	8.3
More than six months and twelve months or less below amortized cost, Fair Value	62.1	1,019.6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.6	40.6
More than twelve months below amortized cost, Fair Value	760.1	377.9
More than twelve months below amortized cost, Unrealized Capital Loss	26.9	31.5
Total, Fair Value	1,088.4	2,316.6
Total Unrealized Capital Losses	28.8	80.4
Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	114.1	235.8
Six months or less below amortized cost, Unrealized Capital Loss	0.7	3.3
More than six months and twelve months or less below amortized cost, Fair Value	3.5	0
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.5	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	117.6	242.0
Total Unrealized Capital Losses	1.2	3.5
Other asset-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	236.7	150.6
Six months or less below amortized cost, Unrealized Capital Loss	0.6	0.9
More than six months and twelve months or less below amortized cost, Fair Value	2.6	105.5
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.1	1.5
More than twelve months below amortized cost, Fair Value	247.7	299.3
More than twelve months below amortized cost, Unrealized Capital Loss	19.5	35.6
Total, Fair Value	487.0	555.4
Total Unrealized Capital Losses	$ 20.2	$ 38.0