Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Sep. 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	$ 17.7	$ 201.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	35.4	4.0
More than twelve months below amortized cost, Unrealized Capital Loss	60.8	73.8
Total Unrealized Capital Losses	113.9	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	1,703.9	6,702.7
Six months or less below amortized cost, Unrealized Capital Loss	$ 21.4	$ 215.8
Six months or less below amortized cost, Number of Securities | securities	355	1,098
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 1,244.8	$ 269.4
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 35.8	$ 14.2
More than six months and twelve months or less below amortized cost, Number of Securities | securities	225	54
More than twelve months below amortized cost, Amortized Cost	$ 739.1	$ 312.8
More than twelve months below amortized cost, Unrealized Capital Loss	$ 29.9	$ 20.7
More than twelve months below amortized cost, Number of Securities | securities	177	128
Total Amortized Cost	$ 3,687.8	$ 7,284.9
Total Unrealized Capital Losses	$ 87.1	$ 250.7
Total Number of Securities | securities	757	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	$ 7.7	$ 22.0
Six months or less below amortized cost, Unrealized Capital Loss	$ 2.1	$ 5.0
Six months or less below amortized cost, Number of Securities | securities	6	9
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 9.9	$ 3.3
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 2.6	$ 0.8
More than six months and twelve months or less below amortized cost, Number of Securities | securities	5	2
More than twelve months below amortized cost, Amortized Cost	$ 71.9	$ 80.8
More than twelve months below amortized cost, Unrealized Capital Loss	$ 22.1	$ 23.0
More than twelve months below amortized cost, Number of Securities | securities	4	5
Total Amortized Cost	$ 89.5	$ 106.1
Total Unrealized Capital Losses	$ 26.8	$ 28.8
Total Number of Securities | securities	15	16
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 0.0	$ 1.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	0.2	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.1	0.0
Total Unrealized Capital Losses	0.3	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	66.5	136.1
Total Unrealized Capital Losses	$ 0.3	$ 1.7
Total Number of Securities | securities	18	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	$ 0.6	$ 14.9
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	2.4	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	1.9	0.7
Total Unrealized Capital Losses	4.9	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	179.1	447.1
Total Unrealized Capital Losses	$ 4.9	$ 15.6
Total Number of Securities | securities	110	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	$ 4.0	$ 53.2
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	6.5	0.2
More than twelve months below amortized cost, Unrealized Capital Loss	12.0	18.6
Total Unrealized Capital Losses	22.5	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	751.0	2,351.7
Total Unrealized Capital Losses	$ 19.2	$ 69.5
Total Number of Securities | securities	160	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	$ 12.7	$ 9.6
Total Unrealized Capital Losses	$ 3.3	$ 2.5
Total Number of Securities | securities	5	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	$ 2.0	$ 48.2
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	12.0	1.0
More than twelve months below amortized cost, Unrealized Capital Loss	26.3	30.5
Total Unrealized Capital Losses	40.3	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	743.3	1,162.2
Total Unrealized Capital Losses	$ 19.5	$ 56.8
Total Number of Securities | securities	60	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	$ 20.8	$ 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	$ 0.6	$ 20.4
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	8.6	17.1
Total Unrealized Capital Losses	10.8	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	200.7	836.9
Total Unrealized Capital Losses	$ 8.2	$ 34.6
Total Number of Securities | securities	44	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	$ 9.9	$ 12.8
Total Unrealized Capital Losses	$ 2.6	$ 3.0
Total Number of Securities | securities	2	3
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 5.1	$ 22.8
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	2.7	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	5.2	3.1
Total Unrealized Capital Losses	13.0	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	381.5	670.4
Total Unrealized Capital Losses	$ 13.0	$ 25.9
Total Number of Securities | securities	36	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	$ 1.8	$ 23.5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	6.4	2.7
More than twelve months below amortized cost, Unrealized Capital Loss	6.6	2.8
Total Unrealized Capital Losses	14.8	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	780.6	1,073.6
Total Unrealized Capital Losses	$ 14.8	$ 29.0
Total Number of Securities | securities	173	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	$ 3.3	$ 16.7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	3.4	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.0	0.0
Total Unrealized Capital Losses	6.7	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	446.7	442.2
Total Unrealized Capital Losses	$ 6.7	$ 16.7
Total Number of Securities | securities	107	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	0.2	0.0
More than twelve months below amortized cost, Unrealized Capital Loss	0.1	1.0
Total Unrealized Capital Losses	0.6	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	138.4	164.7
Total Unrealized Capital Losses	$ 0.5	$ 0.9
Total Number of Securities | securities	49	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	$ 0.5	$ 1.7
Total Unrealized Capital Losses	$ 0.1	$ 0.4
Total Number of Securities | securities	1	1