Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Sep. 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	$ 9.6	$ 160.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	3.6	261.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	86.3	132.9
Total Unrealized Capital Losses	99.5	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	1,123.0	3,980.3
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 26.8	$ 141.7
Six months or less below amortized cost, Number of Securities | securities	175	762
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 176.8	$ 3,001.4
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 6.4	$ 156.6
More than six months and twelve months or less below amortized cost, Number of Securities | securities	52	485
More than twelve months below amortized cost, Amortized Cost	$ 326.4	$ 382.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 15.9	$ 26.9
More than twelve months below amortized cost, Number of Securities | securities	122	144
Total Amortized Cost	$ 1,626.2	$ 7,364.2
Total Unrealized Capital Losses	$ 49.1	$ 325.2
Total Number of Securities | securities	349	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	$ 1.9	$ 211.4
Six months or less below amortized cost, Number of Securities | securities	6	104
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 126.1	$ 27.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 34.2	$ 13.4
More than six months and twelve months or less below amortized cost, Number of Securities | securities	13	2
More than twelve months below amortized cost, Amortized Cost	$ 41.7	$ 17.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 14.3	$ 4.2
More than twelve months below amortized cost, Number of Securities | securities	4	2
Total Amortized Cost	$ 176.5	$ 792.4
Total Unrealized Capital Losses	$ 50.4	$ 229.0
Total Number of Securities | securities	23	108
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.1	$ 0.3
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	0.1	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	30.2	69.7
Total Unrealized Capital Losses	$ 0.1	$ 0.3
Total Number of Securities | securities	5	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	$ 0.8	$ 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	1.1	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	49.9	349.5
Total Unrealized Capital Losses	$ 1.1	$ 7.9
Total Number of Securities | securities	34	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	$ 2.8	$ 67.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	136.4
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	21.2	52.8
Total Unrealized Capital Losses	24.3	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	464.7	3,565.2
Total Unrealized Capital Losses	$ 14.0	$ 153.5
Total Number of Securities | securities	87	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	$ 40.6	$ 373.5
Total Unrealized Capital Losses	$ 10.3	$ 103.3
Total Number of Securities | securities	4	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	$ 2.2	$ 10.9
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.0	43.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	24.1	7.7
Total Unrealized Capital Losses	26.3	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	201.0	791.0
Total Unrealized Capital Losses	$ 7.9	$ 34.6
Total Number of Securities | securities	19	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	$ 18.4	$ 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	1.3	55.8
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	32.9	61.9
Total Unrealized Capital Losses	34.8	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	205.7	1,211.9
Total Unrealized Capital Losses	$ 13.8	$ 63.6
Total Number of Securities | securities	45	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	$ 67.0	$ 291.1
Total Unrealized Capital Losses	$ 21.0	$ 87.9
Total Number of Securities | securities	9	40
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 1.8	$ 42.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	16.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.6	4.9
Total Unrealized Capital Losses	5.7	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	149.1	807.3
Total Unrealized Capital Losses	$ 5.7	$ 53.9
Total Number of Securities | securities	18	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	$ 0.0	$ 35.1
Total Unrealized Capital Losses	$ 0.0	$ 9.6
Total Number of Securities | securities	2	5
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 1.2	$ 1.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.6	0.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.1	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	446.9	294.1
Total Unrealized Capital Losses	$ 5.7	$ 6.9
Total Number of Securities | securities	124	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	$ 0.7	$ 0.0
Total Unrealized Capital Losses	$ 0.2	$ 0.0
Total Number of Securities | securities	4	0
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0.1	$ 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	0.2	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	46.0	239.2
Total Unrealized Capital Losses	$ 0.1	$ 3.5
Total Number of Securities | securities	6	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	$ 0.1	$ 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	1.1	1.3
Total Unrealized Capital Losses	1.1	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	32.7	36.3
Total Unrealized Capital Losses	$ 0.7	$ 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	$ 1.7	$ 1.8
Total Unrealized Capital Losses	$ 0.4	$ 0.4
Total Number of Securities | securities	1	1