Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 39.6
Included in total net realized and unrealized investment gains	0.1	$ 0.1
Included in other comprehensive income-net appreciation on available-for-sale securities	0.4	(0.7)
Sales	(1.5)	(1.7)
Ending Balance	38.6	42.1
Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		44.4
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	21.1
Included in total net realized and unrealized investment gains	0.1	0.1
Included in other comprehensive income-net appreciation on available-for-sale securities	0.2	(0.5)
Sales	(0.9)	(1.1)
Ending Balance	20.5	23.1
Municipal [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		24.6
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.8
Sales	(0.1)	(0.1)
Ending Balance	0.7	0.8
Corporate [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		0.9
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	13.1
Included in other comprehensive income-net appreciation on available-for-sale securities	0.2	(0.2)
Sales	(0.5)	(0.5)
Ending Balance	12.8	13.5
Commercial Mortgage Backed [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		14.2
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	35.0
Included in total net realized and unrealized investment gains	0.1	0.1
Included in other comprehensive income-net appreciation on available-for-sale securities	0.4	(0.7)
Sales	(1.5)	(1.7)
Ending Balance	34.0	37.4
Fixed Maturities [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		39.7
Equity Securities and Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	4.6
Ending Balance	$ 4.6	4.7
Equity Securities and Other Investments [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		$ 4.7