Investments - Unrealized Capital Losses 1 (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 28.7	$ 56.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	23.6	2.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	27.7	53.8
Total Unrealized Capital Losses	80.0	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	1,055.6	1,690.4
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	36.1	50.5
Six months or less below amortized cost, Number of Securities	221	341
More than six months and twelve months or less below amortized cost, Amortized Cost	204.9	115.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	14.2	6.7
More than six months and twelve months or less below amortized cost, Number of Securities	42	34
More than twelve months below amortized cost, Amortized Cost	443.4	1,220.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	18.8	41.8
More than twelve months below amortized cost, Number of Securities	133	223
Total Amortized Cost	1,703.9	3,026.0
Total Unrealized Capital Losses	69.1	99.0
Total Number of Securities	396	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	40.0	59.7
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	10.5	13.2
Six months or less below amortized cost, Number of Securities	7	13
More than six months and twelve months or less below amortized cost, Amortized Cost	0	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More than six months and twelve months or less below amortized cost, Number of Securities	0	0
More than twelve months below amortized cost, Amortized Cost	1.9	2.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.4	0.6
More than twelve months below amortized cost, Number of Securities	1	2
Total Amortized Cost	41.9	61.9
Total Unrealized Capital Losses	10.9	13.8
Total Number of Securities	8	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
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total Unrealized Capital Losses	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	12.4
Total Unrealized Capital Losses	0	0
Total Number of 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
Total Unrealized Capital Losses	0	0
Total Number of 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
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total Unrealized Capital Losses	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	2.3
Total Unrealized Capital Losses	0	0
Total Number of 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
Total Unrealized Capital Losses	0	0
Total Number of 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	0.1	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total Unrealized Capital Losses	0.1	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	13.7	22.6
Total Unrealized Capital Losses	0.1	0.1
Total Number of Securities	4	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
Total Unrealized Capital Losses	0	0
Total Number of Securities	0	0
U.S. corporate securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	12.7	20.0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	8.5	1.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	13.6	27.6
Total Unrealized Capital Losses	34.8	49.1
U.S. corporate securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	795.0	1,543.7
Total Unrealized Capital Losses	31.3	43.4
Total Number of Securities	136	254
U.S. corporate securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	15.5	25.1
Total Unrealized Capital Losses	3.5	5.7
Total Number of Securities	2	5
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	14.9	35.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	15.1	0.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	9.2	20.1
Total Unrealized Capital Losses	39.2	56.0
Foreign | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	662.3	1,052.3
Total Unrealized Capital Losses	32.3	48.5
Total Number of Securities	129	185
Foreign | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	24.5	34.6
Total Unrealized Capital Losses	6.9	7.5
Total Number of Securities	4	6
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.9	0.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	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	200.6	288.6
Total Unrealized Capital Losses	4.3	5.8
Total Number of Securities	109	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
Total Unrealized Capital Losses	0	0
Total Number of 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	0.1	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total Unrealized Capital Losses	0.1	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	13.3	59.2
Total Unrealized Capital Losses	0.1	0.1
Total Number of Securities	6	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
Total Unrealized Capital Losses	0	0
Total Number of 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	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1.5	1.6
Total Unrealized Capital Losses	1.5	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	19.0	44.9
Total Unrealized Capital Losses	1.0	1.1
Total Number of Securities	12	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	1.9	2.2
Total Unrealized Capital Losses	$ 0.5	$ 0.6
Total Number of Securities	1	2