Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014 security	Dec. 31, 2013 security
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	$ 342.7	$ 1,213.2
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(10.5)	(53.2)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	425.9	199.9
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(27.7)	(28.1)
Available-for-sale securities, continuous unrealized loss position, fair value	768.6	1,413.1
U.S. government and agency	(38.2)	(81.3)
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, less than twelve months, fair value	42.4	33.7
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, less than twelve months, accumulated loss	(2.4)	(1.9)
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, twelve months or longer, fair value	17.1	12.1
Available-for-sale securities, continuous unrealized loss position, amounts outside closed block, below investment grade, twelve months or longer, accumulated loss	(2.7)	(1.7)
Available-for-sale securities, continuous unrealized loss position, amount outside closed block, below investment grade, fair value, total	59.5	45.8
Below investment grade	(5.1)	(3.6)
Available-for-sale securities, continuous unrealized loss position, less than 12 Months, number of securities	121	326
Available-for-sale securities, continuous unrealized loss position, 12 Months or longer, number of securities	127	79
Number of securities	248	405
U.S. government and agency
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	0	14.5
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	0	(0.8)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	0	0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	0	0
Available-for-sale securities, continuous unrealized loss position, fair value	0	14.5
U.S. government and agency	0	(0.8)
State and political subdivision
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	7.0	65.3
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(0.4)	(4.4)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	18.7	4.0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(1.2)	(1.0)
Available-for-sale securities, continuous unrealized loss position, fair value	25.7	69.3
U.S. government and agency	(1.6)	(5.4)
Foreign government
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	11.3	17.0
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(0.8)	(0.3)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	0	0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	0	0
Available-for-sale securities, continuous unrealized loss position, fair value	11.3	17.0
U.S. government and agency	(0.8)	(0.3)
Corporate
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	265.4	786.0
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(8.8)	(36.8)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	273.1	120.5
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(18.4)	(18.8)
Available-for-sale securities, continuous unrealized loss position, fair value	538.5	906.5
U.S. government and agency	(27.2)	(55.6)
Commercial mortgage-backed (“CMBS”)
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	6.3	39.4
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	0	(1.1)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	5.5	3.0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	0	(0.1)
Available-for-sale securities, continuous unrealized loss position, fair value	11.8	42.4
U.S. government and agency	0	(1.2)
Residential mortgage-backed (“RMBS”)
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	4.6	243.5
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(0.1)	(8.5)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	86.7	40.3
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(3.3)	(3.3)
Available-for-sale securities, continuous unrealized loss position, fair value	91.3	283.8
U.S. government and agency	(3.4)	(11.8)
Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	42.3	30.5
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(0.4)	(0.3)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	30.2	24.1
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(0.7)	(1.1)
Available-for-sale securities, continuous unrealized loss position, fair value	72.5	54.6
U.S. government and agency	(1.1)	(1.4)
Other asset-backed (“ABS”)
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	5.8	7.9
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	0	(0.1)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	7.5	8.0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(3.8)	(3.8)
Available-for-sale securities, continuous unrealized loss position, fair value	13.3	15.9
U.S. government and agency	(3.8)	(3.9)
Total available-for-sale debt securities
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	342.7	1,204.1
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(10.5)	(52.3)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	421.7	199.9
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(27.4)	(28.1)
Available-for-sale securities, continuous unrealized loss position, fair value	764.4	1,404.0
U.S. government and agency	(37.9)	(80.4)
Available-for-sale securities, continuous unrealized loss position, 12 Months or longer, number of securities	123	79
Equity Securities
Debt Instrument [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	0	9.1
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	0	(0.9)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	4.2	0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(0.3)	0
Available-for-sale securities, continuous unrealized loss position, fair value	4.2	9.1
U.S. government and agency	$ (0.3)	$ (0.9)
Available-for-sale securities, continuous unrealized loss position, 12 Months or longer, number of securities	4	0