Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Jun. 30, 2016 USD ($) securities	Dec. 31, 2015 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 4.1	$ 160.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	12.2	261.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	124.3	132.9
Total Unrealized Capital Losses	140.6	554.2
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	789.6	3,980.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 38.3	$ 141.7
Six months or less below amortized cost, Number of Securities | securities	157	762
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 187.1	$ 3,001.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 7.5	$ 156.6
More than six months and twelve months or less below amortized cost, Number of Securities | securities	53	485
More than twelve months below amortized cost, Amortized Cost	$ 519.0	$ 382.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 22.2	$ 26.9
More than twelve months below amortized cost, Number of Securities | securities	154	144
Total Amortized Cost	$ 1,495.7	$ 7,364.2
Total Unrealized Capital Losses	$ 68.0	$ 325.2
Total Number of Securities | securities	364	1,391
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	$ 8.7	$ 747.5
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 2.2	$ 211.4
Six months or less below amortized cost, Number of Securities | securities	12	104
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 210.5	$ 27.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 59.5	$ 13.4
More than six months and twelve months or less below amortized cost, Number of Securities | securities	15	2
More than twelve months below amortized cost, Amortized Cost	$ 29.3	$ 17.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 10.9	$ 4.2
More than twelve months below amortized cost, Number of Securities | securities	3	2
Total Amortized Cost	$ 248.5	$ 792.4
Total Unrealized Capital Losses	$ 72.6	$ 229.0
Total Number of Securities | securities	30	108
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.0	$ 0.3
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	0.3
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	69.7
Total Unrealized Capital Losses	$ 0.0	$ 0.3
Total Number of Securities | securities	0	14
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
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.0	$ 2.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	5.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.3	0.0
Total Unrealized Capital Losses	0.3	7.9
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	6.7	349.5
Total Unrealized Capital Losses	$ 0.3	$ 7.9
Total Number of Securities | securities	3	117
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	$ 1.4	$ 67.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	4.2	136.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	34.4	52.8
Total Unrealized Capital Losses	40.0	256.8
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	611.9	3,565.2
Total Unrealized Capital Losses	$ 26.3	$ 153.5
Total Number of Securities | securities	132	651
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	$ 52.7	$ 373.5
Total Unrealized Capital Losses	$ 13.7	$ 103.3
Total Number of Securities | securities	5	58
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.3	$ 10.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5	43.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	30.7	7.7
Total Unrealized Capital Losses	31.5	62.4
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	138.0	791.0
Total Unrealized Capital Losses	$ 9.5	$ 34.6
Total Number of Securities | securities	15	87
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	$ 66.4	$ 90.9
Total Unrealized Capital Losses	$ 22.0	$ 27.8
Total Number of Securities | securities	2	4
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	$ 0.6	$ 33.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.7	55.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	51.6	61.9
Total Unrealized Capital Losses	54.9	151.5
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	294.0	1,211.9
Total Unrealized Capital Losses	$ 20.4	$ 63.6
Total Number of Securities | securities	72	254
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	$ 120.0	$ 291.1
Total Unrealized Capital Losses	$ 34.5	$ 87.9
Total Number of Securities | securities	11	40
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.0	$ 42.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.4	16.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.2	4.9
Total Unrealized Capital Losses	6.6	63.5
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	89.0	807.3
Total Unrealized Capital Losses	$ 4.9	$ 53.9
Total Number of Securities | securities	15	85
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	$ 6.5	$ 35.1
Total Unrealized Capital Losses	$ 1.7	$ 9.6
Total Number of Securities | securities	4	5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 1.8	$ 1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.3	0.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	2.8	4.3
Total Unrealized Capital Losses	5.9	6.9
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	301.0	294.1
Total Unrealized Capital Losses	$ 5.7	$ 6.9
Total Number of Securities | securities	111	130
Residential mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.8	$ 0.0
Total Unrealized Capital Losses	$ 0.2	$ 0.0
Total Number of Securities | securities	5	0
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.0	$ 1.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	2.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	0.1	3.5
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	30.4	239.2
Total Unrealized Capital Losses	$ 0.1	$ 3.5
Total Number of Securities | securities	5	38
Commercial mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.1	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	1	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	1.3	1.3
Total Unrealized Capital Losses	1.3	1.4
Other asset-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	24.7	36.3
Total Unrealized Capital Losses	$ 0.8	$ 1.0
Total Number of Securities | securities	11	15
Other asset-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 2.0	$ 1.8
Total Unrealized Capital Losses	$ 0.5	$ 0.4
Total Number of Securities | securities	2	1