Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3), Including Assets Held-for-Sale of Chaucer Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 43.2	$ 51.3	$ 46.6	$ 55.8
Transfers out of Level 3	(0.5)	(2.3)	(0.5)	(2.3)
Included in total net realized and unrealized investment gains			0.1	0.3
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.2)	0.2	(1.1)	0.5
Sales	(0.6)	(0.6)	(3.2)	(5.7)
Balance, Including held-for-sale	41.9	48.6	41.9	48.6
Balance, Excluding held-for-sale	39.7	46.4	39.7	46.4
Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance, Held-for-sale (Chaucer)	2.2	2.2	2.2	2.2
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	24.4	30.1	26.8	31.0
Transfers out of Level 3	(0.5)	(1.9)	(0.5)	(1.9)
Included in total net realized and unrealized investment gains			0.1
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities		0.2	(0.6)	0.6
Sales	(0.4)	(0.3)	(2.3)	(1.6)
Balance, Including held-for-sale	23.5	28.1	23.5	28.1
Balance, Excluding held-for-sale	21.3	25.9	21.3	25.9
Municipal [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance, Held-for-sale (Chaucer)	2.2	2.2	2.2	2.2
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.8	1.0	0.9	4.2
Included in total net realized and unrealized investment gains				0.3
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities				(0.2)
Sales	(0.1)	(0.1)	(0.2)	(3.4)
Balance, Including held-for-sale	0.7	0.9	0.7	0.9
Balance, Excluding held-for-sale	0.7	0.9	0.7	0.9
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	13.3	14.6	14.2	15.0
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.2)		(0.5)	0.1
Sales	(0.1)	(0.2)	(0.7)	(0.7)
Balance, Including held-for-sale	13.0	14.4	13.0	14.4
Balance, Excluding held-for-sale	13.0	14.4	13.0	14.4
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	38.5	45.7	41.9	50.2
Transfers out of Level 3	(0.5)	(1.9)	(0.5)	(1.9)
Included in total net realized and unrealized investment gains			0.1	0.3
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.2)	0.2	(1.1)	0.5
Sales	(0.6)	(0.6)	(3.2)	(5.7)
Balance, Including held-for-sale	37.2	43.4	37.2	43.4
Balance, Excluding held-for-sale	35.0	41.2	35.0	41.2
Fixed Maturities [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance, Held-for-sale (Chaucer)	2.2	2.2	2.2	2.2
Equity Securities and Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	4.7	5.6	4.7	5.6
Transfers out of Level 3		(0.4)		(0.4)
Balance, Including held-for-sale	4.7	5.2	4.7	5.2
Balance, Excluding held-for-sale	$ 4.7	$ 5.2	$ 4.7	$ 5.2