Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains (losses)	$ (33.9)	$ (17.4)
Income tax expense (benefit)	(1.9)	(1.9)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)	0.9	2.7
Net Unrealized Gains / (Losses) on Investments where Credit-related OTTI was Recognized [1] | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains (losses)	1.0	0.2
Total before income taxes	1.0	0.2
Income tax expense (benefit)	0.4	0.1
Net income (loss)	0.6	0.1
Accumulated Other-than-Temporary Impairment | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains (losses)	0.4	4.0
Total before income taxes	0.4	4.0
Income tax expense (benefit)	0.1	1.4
Net income (loss)	$ 0.3	$ 2.6