Investing Activities - Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Fair Value and Cost of Securities:
Amortized Cost	$ 3,114.2	$ 2,798.1
Gross Unrealized Gains	121.2	219.0
Gross Unrealized Losses [1]	(73.2)	(44.8)
Fair Value	3,162.2	2,972.3
OTTI Recognized in AOCI	(13.7)	(14.6)
U.S. government and agency
Fair Value and Cost of Securities:
Amortized Cost	58.0	35.8
Gross Unrealized Gains	3.6	4.8
Gross Unrealized Losses [1]	(0.6)	(0.4)
Fair Value	61.0	40.2
OTTI Recognized in AOCI	0	0
State and political subdivision
Fair Value and Cost of Securities:
Amortized Cost	122.7	120.5
Gross Unrealized Gains	5.5	10.9
Gross Unrealized Losses [1]	(3.3)	(0.5)
Fair Value	124.9	130.9
OTTI Recognized in AOCI	(0.2)	(0.2)
Foreign government
Fair Value and Cost of Securities:
Amortized Cost	42.4	44.4
Gross Unrealized Gains	3.3	7.4
Gross Unrealized Losses [1]	(0.2)	0
Fair Value	45.5	51.8
OTTI Recognized in AOCI	0	0
Corporate
Fair Value and Cost of Securities:
Amortized Cost	2,029.0	1,776.9
Gross Unrealized Gains	77.9	144.6
Gross Unrealized Losses [1]	(51.2)	(24.0)
Fair Value	2,055.7	1,897.5
OTTI Recognized in AOCI	(1.5)	(1.5)
Commercial mortgage-backed (CMBS)
Fair Value and Cost of Securities:
Amortized Cost	239.0	229.6
Gross Unrealized Gains	13.9	26.0
Gross Unrealized Losses [1]	(1.5)	(1.6)
Fair Value	251.4	254.0
OTTI Recognized in AOCI	(0.5)	(0.6)
Residential mortgage-backed (RMBS)
Fair Value and Cost of Securities:
Amortized Cost	457.7	412.6
Gross Unrealized Gains	10.8	17.5
Gross Unrealized Losses [1]	(8.7)	(7.6)
Fair Value	459.8	422.5
OTTI Recognized in AOCI	(8.5)	(9.0)
CDO/CLO
Fair Value and Cost of Securities:
Amortized Cost	67.8	59.2
Gross Unrealized Gains	2.1	1.8
Gross Unrealized Losses [1]	(2.2)	(4.2)
Fair Value	67.7	56.8
OTTI Recognized in AOCI	(3.0)	(3.3)
Other asset-backed
Fair Value and Cost of Securities:
Amortized Cost	97.6	119.1
Gross Unrealized Gains	4.1	6.0
Gross Unrealized Losses [1]	(5.5)	(6.5)
Fair Value	96.2	118.6
OTTI Recognized in AOCI	$ 0	$ 0