Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 29.4	$ 31.0
Included in total net realized and unrealized investment gains (losses)	0.1
Included in other comprehensive loss-net appreciation (depreciation) on available-for-sale securities	(0.4)	(0.3)
Purchases and Sales:
Sales	(1.1)	(1.4)
Ending Balance	28.0	29.3
Change in unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period	(0.4)	(0.3)
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	7.0	12.1
Included in other comprehensive loss-net appreciation (depreciation) on available-for-sale securities		0.1
Purchases and Sales:
Sales	(0.8)	(0.8)
Ending Balance	6.2	11.4
Change in unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period		0.1
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.5	0.6
Purchases and Sales:
Sales	(0.1)	(0.1)
Ending Balance	0.4	0.5
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	12.4	12.7
Included in other comprehensive loss-net appreciation (depreciation) on available-for-sale securities	(0.4)	(0.4)
Purchases and Sales:
Sales	(0.2)	(0.5)
Ending Balance	11.8	11.8
Change in unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period	(0.4)	(0.4)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	19.9	25.4
Included in other comprehensive loss-net appreciation (depreciation) on available-for-sale securities	(0.4)	(0.3)
Purchases and Sales:
Sales	(1.1)	(1.4)
Ending Balance	18.4	23.7
Change in unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period	(0.4)	(0.3)
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	9.5	5.6
Included in total net realized and unrealized investment gains (losses)	0.1
Purchases and Sales:
Ending Balance	$ 9.6	$ 5.6