Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Sep. 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	$ 216.1	$ 56.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	67.2	2.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	85.4	53.8
Total Unrealized Capital Losses	368.7	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	4,550.6	1,690.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 191.9	$ 50.5
Six months or less below amortized cost, Number of Securities | securities	804	341
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 653.5	$ 115.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 42.6	$ 6.7
More than six months and twelve months or less below amortized cost, Number of Securities | securities	110	34
More than twelve months below amortized cost, Amortized Cost	$ 424.8	$ 1,220.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 28.6	$ 41.8
More than twelve months below amortized cost, Number of Securities | securities	142	223
Total Amortized Cost	$ 5,628.9	$ 3,026.0
Total Unrealized Capital Losses	$ 263.1	$ 99.0
Total Number of Securities | securities	1,056	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	$ 328.1	$ 59.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 96.3	$ 13.2
Six months or less below amortized cost, Number of Securities | securities	52	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	$ 3.6	$ 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	$ 22.9	$ 2.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 5.7	$ 0.6
More than twelve months below amortized cost, Number of Securities | securities	2	2
Total Amortized Cost	$ 366.5	$ 61.9
Total Unrealized Capital Losses	$ 105.6	$ 13.8
Total Number of Securities | securities	56	15
U.S. Treasuries
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. Treasuries | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0.0	12.4
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	0	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	$ 5.3	$ 0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.8	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	6.1	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	230.3	22.6
Total Unrealized Capital Losses	$ 6.1	$ 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	$ 101.1	$ 18.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	32.2	1.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	34.8	20.8
Total Unrealized Capital Losses	168.1	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	2,779.7	1,270.1
Total Unrealized Capital Losses	$ 125.9	$ 38.1
Total Number of Securities | securities	526	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	$ 142.2	$ 9.6
Total Unrealized Capital Losses	$ 42.2	$ 2.1
Total Number of Securities | securities	22	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	$ 30.7	$ 2.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.2	0.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	7.1	6.8
Total Unrealized Capital Losses	41.0	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	603.4	273.6
Total Unrealized Capital Losses	$ 37.5	$ 5.3
Total Number of Securities | securities	50	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	$ 15.5	$ 15.5
Total Unrealized Capital Losses	$ 3.5	$ 3.6
Total Number of Securities | securities	1	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	$ 55.6	$ 33.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	28.3	0.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	36.7	16.6
Total Unrealized Capital Losses	120.6	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	1,025.6	903.6
Total Unrealized Capital Losses	$ 62.4	$ 44.5
Total Number of Securities | securities	207	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	$ 202.2	$ 26.2
Total Unrealized Capital Losses	$ 58.2	$ 5.8
Total Number of Securities | securities	30	5
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 21.6	$ 2.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.5	0.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.0	3.5
Total Unrealized Capital Losses	26.1	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	645.7	148.7
Total Unrealized Capital Losses	$ 24.8	$ 4.0
Total Number of Securities | securities	68	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	$ 4.8	$ 8.4
Total Unrealized Capital Losses	$ 1.3	$ 1.7
Total Number of Securities | securities	1	1
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.7	$ 0.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	0.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.4	4.5
Total Unrealized Capital Losses	4.3	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	208.1	288.6
Total Unrealized Capital Losses	$ 4.3	$ 5.8
Total Number of Securities | securities	106	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	$ 1.1	$ 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	1.1	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	109.2	59.2
Total Unrealized Capital Losses	$ 1.1	$ 0.1
Total Number of Securities | securities	15	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	26.9	44.9
Total Unrealized Capital Losses	$ 1.0	$ 1.1
Total Number of Securities | securities	14	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	$ 0.4	$ 0.6
Total Number of Securities | securities	1	2