Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Jun. 30, 2015 USD ($) securities	Dec. 31, 2014 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 162.8	$ 56.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	44.1	2.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	37.5	53.8
Total Unrealized Capital Losses	244.4	112.8
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	5,009.8	1,690.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 166.0	$ 50.5
Six months or less below amortized cost, Number of Securities | securities	841	341
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 344.3	$ 115.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 28.4	$ 6.7
More than six months and twelve months or less below amortized cost, Number of Securities | securities	75	34
More than twelve months below amortized cost, Amortized Cost	$ 407.2	$ 1,220.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 24.2	$ 41.8
More than twelve months below amortized cost, Number of Securities | securities	131	223
Total Amortized Cost	$ 5,761.3	$ 3,026.0
Total Unrealized Capital Losses	$ 218.6	$ 99.0
Total Number of Securities | securities	1,047	598
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	$ 85.8	$ 59.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 21.5	$ 13.2
Six months or less below amortized cost, Number of Securities | securities	9	13
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 15.5	$ 0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 3.8	$ 0.0
More than six months and twelve months or less below amortized cost, Number of Securities | securities	2	0
More than twelve months below amortized cost, Amortized Cost	$ 1.9	$ 2.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 0.5	$ 0.6
More than twelve months below amortized cost, Number of Securities | securities	1	2
Total Amortized Cost	$ 103.2	$ 61.9
Total Unrealized Capital Losses	$ 25.8	$ 13.8
Total Number of Securities | securities	12	15
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.1	$ 0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	0.1	0.0
U.S. Treasuries | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	15.9	12.4
Total Unrealized Capital Losses	$ 0.1	$ 0.0
Total Number of Securities | securities	2	1
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	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | 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	$ 0.0	$ 0.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	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
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	0.0	2.3
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	0	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	$ 0.0	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	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	$ 8.5	$ 0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	8.5	0.1
State, municipalities and political subdivisions | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	227.4	22.6
Total Unrealized Capital Losses	$ 8.5	$ 0.1
Total Number of Securities | securities	70	8
State, municipalities and political subdivisions | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	0	0
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 86.4	$ 18.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	13.3	1.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.8	20.8
Total Unrealized Capital Losses	111.5	40.2
U.S. corporate public securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	2,926.7	1,270.1
Total Unrealized Capital Losses	$ 111.5	$ 38.1
Total Number of Securities | securities	525	224
U.S. corporate public securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 9.6
Total Unrealized Capital Losses	$ 0.0	$ 2.1
Total Number of Securities | securities	1	4
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 27.0	$ 2.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.6	0.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.6	6.8
Total Unrealized Capital Losses	34.2	8.9
U.S. corporate private securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	625.1	273.6
Total Unrealized Capital Losses	$ 28.6	$ 5.3
Total Number of Securities | securities	54	30
U.S. corporate private securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 24.5	$ 15.5
Total Unrealized Capital Losses	$ 5.6	$ 3.6
Total Number of Securities | securities	2	1
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	$ 24.6	$ 33.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	27.3	0.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12.3	16.6
Total Unrealized Capital Losses	64.2	50.3
Foreign corporate public securities and foreign governments | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,081.9	903.6
Total Unrealized Capital Losses	$ 44.5	$ 44.5
Total Number of Securities | securities	208	165
Foreign corporate public securities and foreign governments | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 76.9	$ 26.2
Total Unrealized Capital Losses	$ 19.7	$ 5.8
Total Number of Securities | securities	7	5
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 9.0	$ 2.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.3	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.6	3.5
Total Unrealized Capital Losses	11.9	5.7
Foreign corporate private securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	379.6	148.7
Total Unrealized Capital Losses	$ 11.9	$ 4.0
Total Number of Securities | securities	39	20
Foreign corporate private securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 8.4
Total Unrealized Capital Losses	$ 0.0	$ 1.7
Total Number of Securities | securities	0	1
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 1.1	$ 0.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.6	0.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	4.8	4.5
Total Unrealized Capital Losses	6.5	5.8
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	231.2	288.6
Total Unrealized Capital Losses	$ 6.5	$ 5.8
Total Number of Securities | securities	111	124
Residential mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	1	2
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 6.1	$ 0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	6.1	0.1
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	254.8	59.2
Total Unrealized Capital Losses	$ 6.1	$ 0.1
Total Number of Securities | securities	26	11
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	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | 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	$ 0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.4	1.6
Total Unrealized Capital Losses	1.4	1.7
Other asset-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	18.7	44.9
Total Unrealized Capital Losses	$ 0.9	$ 1.1
Total Number of Securities | securities	12	14
Other asset-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 1.8	$ 2.2
Total Unrealized Capital Losses	$ 0.5	$ 0.6
Total Number of Securities | securities	1	2