Investing Activities - Schedule of Temporariliy Impaired Securities (Details) $ in Millions	Jun. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	$ 1,242.2	$ 342.7
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(42.0)	(10.5)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	287.0	425.9
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(28.3)	(27.7)
Available-for-sale securities, continuous unrealized loss position, fair value	1,529.2	768.6
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(70.3)	(38.2)
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, less than twelve months, fair value	65.2	42.4
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, less than 12 months, accumulated loss	(3.2)	(2.4)
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, twelve months or longer, fair value	24.0	17.1
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, 12 months or longer, accumulated loss	(3.5)	(2.7)
Available-for-sale securities, continuous unrealized loss position, amount outside closed block, below investment grade, fair value, total	89.2	59.5
Available-for-sale securities, below investment grade, continuous unrealized loss position	$ (6.7)	$ (5.1)
Available-for-sale securities, securities in unrealized loss positions, less than 12 months, number of securities | security	313	121
Available-for-sale, securities in unrealized loss position, number of positions greater than or equal to one year, number of securities | security	92	127
Available-for-sale, securities in unrealized loss positions, number of securities | security	405	248
U.S. government and agency
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	$ 0.0	$ 0.0
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, fair value	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, accumulated loss	0.0	0.0
State and political subdivision
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	45.8	7.0
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(1.1)	(0.4)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	16.2	18.7
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(1.8)	(1.2)
Available-for-sale securities, continuous unrealized loss position, fair value	62.0	25.7
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(2.9)	(1.6)
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	19.9	11.3
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(0.5)	(0.8)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, fair value	19.9	11.3
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(0.5)	(0.8)
Corporate
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	1,056.9	265.4
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(39.0)	(8.8)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	172.6	273.1
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(18.5)	(18.4)
Available-for-sale securities, continuous unrealized loss position, fair value	1,229.5	538.5
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(57.5)	(27.2)
Commercial mortgage-backed (“CMBS”)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	20.1	6.3
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(0.3)	0.0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	0.0	5.5
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, fair value	20.1	11.8
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(0.3)	0.0
Residential mortgage-backed (“RMBS”)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	36.7	4.6
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(0.6)	(0.1)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	59.8	86.7
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(3.2)	(3.3)
Available-for-sale securities, continuous unrealized loss position, fair value	96.5	91.3
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(3.8)	(3.4)
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	56.4	42.3
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(0.5)	(0.4)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	27.0	30.2
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(0.5)	(0.7)
Available-for-sale securities, continuous unrealized loss position, fair value	83.4	72.5
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(1.0)	(1.1)
Other asset-backed (“ABS”)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	6.4	5.8
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	7.1	7.5
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(4.1)	(3.8)
Available-for-sale securities, continuous unrealized loss position, fair value	13.5	13.3
Available-for-sale securities, continuous unrealized loss position, accumulated loss	(4.1)	(3.8)
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	1,242.2	342.7
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	(42.0)	(10.5)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	282.7	421.7
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(28.1)	(27.4)
Available-for-sale securities, continuous unrealized loss position, fair value	1,524.9	764.4
Available-for-sale securities, continuous unrealized loss position, accumulated loss	$ (70.1)	(37.9)
Available-for-sale, securities in unrealized loss position, number of positions greater than or equal to one year, number of securities | security	88
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	$ 0.0	0.0
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	0.0	0.0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	4.3	4.2
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(0.2)	(0.3)
Available-for-sale securities, continuous unrealized loss position, fair value	4.3	4.2
Available-for-sale securities, continuous unrealized loss position, accumulated loss	$ (0.2)	$ (0.3)
Available-for-sale, securities in unrealized loss position, number of positions greater than or equal to one year, number of securities | security	4