Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			$ 303.8	$ 381.4
Balance, end of period	$ 295.4	$ 390.5	295.4	390.5
Net Unrealized Gains / (Losses) on Investments where Credit-related OTTI was Recognized [1] | Available-for-sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	2.1	3.5	4.4	3.0
Change in component during the period before reclassifications	(0.1)	0.2	(2.3)	1.3
Amounts reclassified from AOCI	0.0	(0.2)	(0.1)	(0.8)
Balance, end of period	2.0	3.5	2.0	3.5
Net Unrealized Gains / (Losses) on Investments where Credit-related OTTI was Recognized [1] | Other Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	0.6	(17.6)	(0.4)	(15.7)
Change in component during the period before reclassifications	(13.6)	29.8	(9.6)	28.2
Amounts reclassified from AOCI	(5.3)	(0.1)	(8.3)	(0.4)
Balance, end of period	(18.3)	12.1	(18.3)	12.1
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS):
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	2.7	(14.1)	4.0	(12.7)
Change in component during the period before reclassifications	(13.7)	30.0	(11.9)	29.5
Amounts reclassified from AOCI	(5.3)	(0.3)	(8.4)	(1.2)
Balance, end of period	$ (16.3)	$ 15.6	$ (16.3)	$ 15.6