Investing Activities - Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value and Cost of Securities:
Amortized Cost	$ 2,882.4	$ 2,798.1
Gross Unrealized Gains	210.6	219.0
Gross Unrealized Losses	(38.3)	(44.8)
Available-for-sale debt securities	3,054.7	2,972.3
OTTI Recognized in AOCI	(13.9)	(14.6)
U.S. government and agency
Fair Value and Cost of Securities:
Amortized Cost	41.1	35.8
Gross Unrealized Gains	4.9	4.8
Gross Unrealized Losses	(0.4)	(0.4)
Available-for-sale debt securities	45.6	40.2
OTTI Recognized in AOCI	0	0
State and political subdivision
Fair Value and Cost of Securities:
Amortized Cost	106.2	120.5
Gross Unrealized Gains	11.2	10.9
Gross Unrealized Losses	(0.3)	(0.5)
Available-for-sale debt securities	117.1	130.9
OTTI Recognized in AOCI	(0.2)	(0.2)
Foreign government
Fair Value and Cost of Securities:
Amortized Cost	44.4	44.4
Gross Unrealized Gains	5.7	7.4
Gross Unrealized Losses	0	0
Available-for-sale debt securities	50.1	51.8
OTTI Recognized in AOCI	0	0
Corporate
Fair Value and Cost of Securities:
Amortized Cost	1,908.2	1,776.9
Gross Unrealized Gains	140.3	144.6
Gross Unrealized Losses	(23.0)	(24.0)
Available-for-sale debt securities	2,025.5	1,897.5
OTTI Recognized in AOCI	(1.5)	(1.5)
Commercial mortgage-backed (CMBS)
Fair Value and Cost of Securities:
Amortized Cost	225.2	229.6
Gross Unrealized Gains	24.1	26.0
Gross Unrealized Losses	(0.6)	(1.6)
Available-for-sale debt securities	248.7	254.0
OTTI Recognized in AOCI	(0.6)	(0.6)
Residential mortgage-backed (RMBS)
Fair Value and Cost of Securities:
Amortized Cost	384.9	412.6
Gross Unrealized Gains	16.6	17.5
Gross Unrealized Losses	(5.5)	(7.6)
Available-for-sale debt securities	396.0	422.5
OTTI Recognized in AOCI	(8.6)	(9.0)
CDO/CLO
Fair Value and Cost of Securities:
Amortized Cost	63.0	59.2
Gross Unrealized Gains	1.8	1.8
Gross Unrealized Losses	(2.6)	(4.2)
Available-for-sale debt securities	62.2	56.8
OTTI Recognized in AOCI	(3.0)	(3.3)
Other asset-backed
Fair Value and Cost of Securities:
Amortized Cost	109.4	119.1
Gross Unrealized Gains	6.0	6.0
Gross Unrealized Losses	(5.9)	(6.5)
Available-for-sale debt securities	109.5	118.6
OTTI Recognized in AOCI	$ 0	$ 0