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	$ 83.5	$ 131.3
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, Unrealized Capital Loss	260.2	205.2
Total Unrealized Capital Losses	349.1	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	1.9	24.3
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, Unrealized Capital Loss	3.2	9.9
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, Unrealized Capital Loss	0.1	0
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, Unrealized Capital Loss	0	0
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, Unrealized Capital Loss	54.3	78.5
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, Unrealized Capital Loss	162.9	109.6
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, Unrealized Capital Loss	24.6	16.0
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, Unrealized Capital Loss	47.7	18.4
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, Unrealized Capital Loss	1.3	8.3
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, Unrealized Capital Loss	26.9	31.5
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, Unrealized Capital Loss	0.7	3.3
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, Unrealized Capital Loss	0	0.2
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, Unrealized Capital Loss	0.6	0.9
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, Unrealized Capital Loss	19.5	35.6
Total Unrealized Capital Losses	20.2	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	6,559.3	7,883.3
Six months or less below amortized cost, Unrealized Capital Loss	85.4	166.0
Six months or less below amortized cost, Number of Securities	559	570
More than six months and twelve months or less below amortized cost, Amortized Cost	362.1	12,339.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	23.7	776.8
More than six months and twelve months or less below amortized cost, Number of Securities	74	798
More than twelve months below amortized cost, Amortized Cost	5,568.4	1,579.2
More than twelve months below amortized cost, Unrealized Capital Loss	235.4	144.5
More than twelve months below amortized cost, Number of Securities	551	302
Total Amortized Cost	12,489.8	21,802.2
Total Unrealized Capital Losses	344.5	1,087.3
Number of Securities	1,184	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	451.0	2,750.5
Total Unrealized Capital Losses	5.1	81.4
Number of Securities	16	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	7.0	66.8
Total Unrealized Capital Losses	0.1	1.4
Number of Securities	1	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	7,734.6	11,892.6
Total Unrealized Capital Losses	220.9	694.2
Number of Securities	548	744
Fair value decline below amortized cost less than 20% | Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	2,567.2	3,944.2
Total Unrealized Capital Losses	72.6	211.0
Number of Securities	197	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	1,113.0	2,361.4
Total Unrealized Capital Losses	26.6	70.2
Number of Securities	323	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	118.8	245.5
Total Unrealized Capital Losses	1.2	3.5
Number of Securities	10	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	498.2	541.2
Total Unrealized Capital Losses	18.0	25.6
Number of Securities	89	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	2.8	80.5
Six months or less below amortized cost, Unrealized Capital Loss	0.6	18.6
Six months or less below amortized cost, Number of Securities	4	20
More than six months and twelve months or less below amortized cost, Amortized Cost	0	67.6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0	16.7
More than six months and twelve months or less below amortized cost, Number of Securities	2	8
More than twelve months below amortized cost, Amortized Cost	11.4	55.8
More than twelve months below amortized cost, Unrealized Capital Loss	4.0	13.9
More than twelve months below amortized cost, Number of Securities	8	22
Total Amortized Cost	14.2	203.9
Total Unrealized Capital Losses	4.6	49.2
Number of Securities	14	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	1.0	55.8
Total Unrealized Capital Losses	0.2	13.5
Number of Securities	2	5
Fair value decline below amortized cost greater than 20% | Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	0	35.0
Total Unrealized Capital Losses	0	7.7
Number of Securities	0	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	4.2	35.6
Total Unrealized Capital Losses	2.2	10.2
Number of Securities	7	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	0
Total Unrealized Capital Losses	0	0
Number of Securities	0	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	9.0	52.2
Total Unrealized Capital Losses	$ 2.2	$ 12.4
Number of Securities	5	13