Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Jun. 30, 2017 USD ($) securities	Dec. 31, 2016 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 10.0	$ 201.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	71.4	4.0
More than twelve months below amortized cost, Unrealized Capital Loss	56.0	73.8
Total Unrealized Capital Losses	137.4	279.5
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	958.9	6,702.7
Six months or less below amortized cost, Unrealized Capital Loss	$ 10.5	$ 215.8
Six months or less below amortized cost, Number of Securities | securities	238	1,098
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 2,584.9	$ 269.4
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 77.9	$ 14.2
More than six months and twelve months or less below amortized cost, Number of Securities | securities	420	54
More than twelve months below amortized cost, Amortized Cost	$ 333.4	$ 312.8
More than twelve months below amortized cost, Unrealized Capital Loss	$ 15.7	$ 20.7
More than twelve months below amortized cost, Number of Securities | securities	127	128
Total Amortized Cost	$ 3,877.2	$ 7,284.9
Total Unrealized Capital Losses	$ 104.1	$ 250.7
Total Number of Securities | securities	785	1,280
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	$ 38.7	$ 22.0
Six months or less below amortized cost, Unrealized Capital Loss	$ 9.5	$ 5.0
Six months or less below amortized cost, Number of Securities | securities	9	9
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 0.0	$ 3.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 0.0	$ 0.8
More than six months and twelve months or less below amortized cost, Number of Securities | securities	4	2
More than twelve months below amortized cost, Amortized Cost	$ 71.8	$ 80.8
More than twelve months below amortized cost, Unrealized Capital Loss	$ 23.8	$ 23.0
More than twelve months below amortized cost, Number of Securities | securities	3	5
Total Amortized Cost	$ 110.5	$ 106.1
Total Unrealized Capital Losses	$ 33.3	$ 28.8
Total Number of Securities | securities	16	16
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 0.2	$ 1.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	0.3	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	0.5	1.7
U.S. Treasuries | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	88.1	136.1
Total Unrealized Capital Losses	$ 0.5	$ 1.7
Total Number of Securities | securities	21	22
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	$ 0.2	$ 14.9
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	5.8	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.5	0.7
Total Unrealized Capital Losses	6.5	15.6
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	216.0	447.1
Total Unrealized Capital Losses	$ 6.5	$ 15.6
Total Number of Securities | securities	115	187
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	$ 2.2	$ 53.2
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	14.9	0.2
More than twelve months below amortized cost, Unrealized Capital Loss	12.0	18.6
Total Unrealized Capital Losses	29.1	72.0
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	893.7	2,351.7
Total Unrealized Capital Losses	$ 22.0	$ 69.5
Total Number of Securities | securities	178	417
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	$ 28.0	$ 9.6
Total Unrealized Capital Losses	$ 7.1	$ 2.5
Total Number of Securities | securities	4	3
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 2.4	$ 48.2
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	18.1	1.0
More than twelve months below amortized cost, Unrealized Capital Loss	26.4	30.5
Total Unrealized Capital Losses	46.9	79.7
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	789.2	1,162.2
Total Unrealized Capital Losses	$ 25.0	$ 56.8
Total Number of Securities | securities	65	108
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	$ 81.9
Total Unrealized Capital Losses	$ 21.9	$ 22.9
Total Number of Securities | securities	2	3
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	$ 1.6	$ 20.4
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	2.0	0.1
More than twelve months below amortized cost, Unrealized Capital Loss	12.6	17.1
Total Unrealized Capital Losses	16.2	37.6
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	307.8	836.9
Total Unrealized Capital Losses	$ 11.9	$ 34.6
Total Number of Securities | securities	69	159
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	$ 16.1	$ 12.8
Total Unrealized Capital Losses	$ 4.3	$ 3.0
Total Number of Securities | securities	5	3
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 0.9	$ 22.8
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	9.0	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.9	3.1
Total Unrealized Capital Losses	10.8	25.9
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	369.7	670.4
Total Unrealized Capital Losses	$ 10.8	$ 25.9
Total Number of Securities | securities	39	64
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	$ 0.0
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	2	2
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 1.0	$ 23.5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	11.9	2.7
More than twelve months below amortized cost, Unrealized Capital Loss	3.4	2.8
Total Unrealized Capital Losses	16.3	29.0
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	734.6	1,073.6
Total Unrealized Capital Losses	$ 16.3	$ 29.0
Total Number of Securities | securities	168	196
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	$ 0.1
Total Unrealized Capital Losses	$ 0.0	$ 0.0
Total Number of Securities | securities	3	3
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 1.2	$ 16.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	9.1	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	10.3	16.7
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	350.3	442.2
Total Unrealized Capital Losses	$ 10.3	$ 16.7
Total Number of Securities | securities	85	90
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	1
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 0.3	$ 0.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	0.3	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.2	1.0
Total Unrealized Capital Losses	0.8	1.3
Other asset-backed securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	127.8	164.7
Total Unrealized Capital Losses	$ 0.8	$ 0.9
Total Number of Securities | securities	45	37
Other asset-backed securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0.0	$ 1.7
Total Unrealized Capital Losses	$ 0.0	$ 0.4
Total Number of Securities | securities	0	1