Investments (excluding Consolidated Investment Entities) - Unrealized Capital Losses 1 (Details) $ in Millions	Jun. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 535.3	$ 148.2
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	107.3	6.7
More than twelve months below amortized cost, Unrealized Capital Loss	109.3	168.7
Total Unrealized Capital Losses	751.9	323.6
U.S. Treasuries
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 Losses	0.0	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.0	0.8
Total Unrealized Capital Losses	0.1	0.9
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	0.0	0.0
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.0	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	0.0	0.0
State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	18.7	0.1
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.0	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.6	0.4
Total Unrealized Capital Losses	19.3	0.5
U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	349.5	60.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	39.1	3.4
More than twelve months below amortized cost, Unrealized Capital Loss	34.5	75.6
Total Unrealized Capital Losses	423.1	139.7
U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	63.4	4.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	4.1	0.2
More than twelve months below amortized cost, Unrealized Capital Loss	16.4	19.7
Total Unrealized Capital Losses	83.9	24.2
Foreign corporate public securities and foreign governments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	66.3	72.5
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	56.7	1.2
More than twelve months below amortized cost, Unrealized Capital Loss	21.5	27.4
Total Unrealized Capital Losses	144.5	101.1
Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	24.6	8.1
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	5.7	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	4.0	8.8
Total Unrealized Capital Losses	34.3	16.9
Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	4.9	1.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	1.2	1.0
More than twelve months below amortized cost, Unrealized Capital Loss	17.1	18.9
Total Unrealized Capital Losses	23.2	21.6
Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	7.6	0.5
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	0.5	0.9
More than twelve months below amortized cost, Unrealized Capital Loss	1.0	0.0
Total Unrealized Capital Losses	9.1	1.4
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 Losses	0.0	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	14.2	17.1
Total Unrealized Capital Losses	14.4	17.3
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	15,565.2	5,162.1
Six months or less below amortized cost, Unrealized Capital Loss	$ 548.3	$ 140.2
Six months or less below amortized cost, Number of Securities | security	1,244	537
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 957.2	$ 324.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	$ 77.5	$ 19.7
More than six months and twelve months or less below amortized cost, Number of Securities | security	121	68
More than twelve months below amortized cost, Amortized Cost	$ 1,339.1	$ 4,237.2
More than twelve months below amortized cost, Unrealized Capital Loss	$ 77.9	$ 134.1
More than twelve months below amortized cost, Number of Securities | security	314	493
Total Amortized Cost	$ 17,861.5	$ 9,723.6
Total Unrealized Capital Losses	$ 703.7	$ 294.0
Number of Securities | security	1,679	1,098
Fair value decline below amortized cost less than 20% | U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 16.6	$ 124.1
Total Unrealized Capital Losses	$ 0.1	$ 0.9
Number of Securities | security	3	8
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	$ 0.0	$ 6.4
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Number of Securities | security	0	1
Fair value decline below amortized cost less than 20% | State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 513.1	$ 44.1
Total Unrealized Capital Losses	$ 18.6	$ 0.2
Number of Securities | security	70	9
Fair value decline below amortized cost less than 20% | U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 10,470.7	$ 4,737.5
Total Unrealized Capital Losses	$ 423.1	$ 137.6
Number of Securities | security	818	383
Fair value decline below amortized cost less than 20% | U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 1,471.0	$ 635.2
Total Unrealized Capital Losses	$ 67.7	$ 13.7
Number of Securities | security	57	31
Fair value decline below amortized cost less than 20% | Foreign corporate public securities and foreign governments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 2,945.6	$ 2,115.0
Total Unrealized Capital Losses	$ 117.5	$ 93.1
Number of Securities | security	274	219
Fair value decline below amortized cost less than 20% | Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 970.6	$ 521.5
Total Unrealized Capital Losses	$ 34.3	$ 12.6
Number of Securities | security	39	20
Fair value decline below amortized cost less than 20% | Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 801.0	$ 1,042.8
Total Unrealized Capital Losses	$ 21.2	$ 19.5
Number of Securities | security	300	321
Fair value decline below amortized cost less than 20% | Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 356.3	$ 121.2
Total Unrealized Capital Losses	$ 8.1	$ 0.5
Number of Securities | security	33	17
Fair value decline below amortized cost less than 20% | Other asset-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 316.6	$ 375.8
Total Unrealized Capital Losses	$ 13.1	$ 15.9
Number of Securities | security	85	89
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	$ 138.3	$ 117.8
Six months or less below amortized cost, Unrealized Capital Loss	$ 32.6	$ 26.5
Six months or less below amortized cost, Number of Securities | security	16	16
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 50.9	$ 0.0
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	$ 12.7	$ 0.0
More than six months and twelve months or less below amortized cost, Number of Securities | security	6	1
More than twelve months below amortized cost, Amortized Cost	$ 8.2	$ 8.6
More than twelve months below amortized cost, Unrealized Capital Loss	$ 2.9	$ 3.1
More than twelve months below amortized cost, Number of Securities | security	5	7
Total Amortized Cost	$ 197.4	$ 126.4
Total Unrealized Capital Losses	$ 48.2	$ 29.6
Number of Securities | security	27	24
Fair value decline below amortized cost greater than 20% | U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Number of Securities | security	0	0
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	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Number of Securities | security	0	0
Fair value decline below amortized cost greater than 20% | State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 2.0	$ 1.0
Total Unrealized Capital Losses	$ 0.7	$ 0.3
Number of Securities | security	3	1
Fair value decline below amortized cost greater than 20% | U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 8.8
Total Unrealized Capital Losses	$ 0.0	$ 2.1
Number of Securities | security	2	3
Fair value decline below amortized cost greater than 20% | U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 70.0	$ 45.0
Total Unrealized Capital Losses	$ 16.2	$ 10.5
Number of Securities | security	2	1
Fair value decline below amortized cost greater than 20% | Foreign corporate public securities and foreign governments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 112.0	$ 36.2
Total Unrealized Capital Losses	$ 27.0	$ 8.0
Number of Securities | security	9	5
Fair value decline below amortized cost greater than 20% | Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 21.0
Total Unrealized Capital Losses	$ 0.0	$ 4.3
Number of Securities | security	0	1
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	$ 4.0
Total Unrealized Capital Losses	$ 2.0	$ 2.1
Number of Securities | security	7	8
Fair value decline below amortized cost greater than 20% | Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 3.2	$ 4.0
Total Unrealized Capital Losses	$ 1.0	$ 0.9
Number of Securities | security	1	1
Fair value decline below amortized cost greater than 20% | Other asset-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 6.0	$ 6.4
Total Unrealized Capital Losses	$ 1.3	$ 1.4
Number of Securities | security	3	4