Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	$ 46.6		$ 53.0
Transfers out of Level 3	(0.5)		(1.9)
Included in total net realized and unrealized investment gains (losses)			0.3
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.5)		(0.9)
Sales	(3.8)		(6.1)
Balance at end of year	39.6	[1]	46.6
Balance, Excluding held-for-sale			44.4
Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	44.4
Balance at end of year			44.4
Balance, Held-for-sale (Chaucer)			2.2
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	26.8		28.8
Transfers out of Level 3	(0.5)		(1.9)
Included in total net realized and unrealized investment gains (losses)	0.1
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.2)		(0.2)
Sales	(2.9)		(2.1)
Balance at end of year	21.1	[1]	26.8
Balance, Excluding held-for-sale			24.6
Municipal [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	24.6
Balance at end of year			24.6
Balance, Held-for-sale (Chaucer)			2.2
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	0.9		4.0
Included in total net realized and unrealized investment gains (losses)			0.3
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities			(0.2)
Sales	(0.1)		(3.2)
Balance at end of year	0.8	[1]	0.9
Balance, Excluding held-for-sale			0.9
Corporate [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	0.9
Balance at end of year			0.9
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	14.2		15.0
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.3)
Sales	(0.8)		(0.8)
Balance at end of year	13.1	[1]	14.2
Balance, Excluding held-for-sale			14.2
Commercial Mortgage Backed [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	14.2
Balance at end of year			14.2
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	41.9		47.8
Transfers out of Level 3	(0.5)		(1.9)
Included in total net realized and unrealized investment gains (losses)	0.1		0.3
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.5)		(0.4)
Sales	(3.8)		(6.1)
Balance at end of year	35.0	[1]	41.9
Balance, Excluding held-for-sale			39.7
Fixed Maturities [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	39.7
Balance at end of year			39.7
Balance, Held-for-sale (Chaucer)			2.2
Equity Securities and Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	4.7		5.2
Included in total net realized and unrealized investment gains (losses)	(0.1)
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities			(0.5)
Balance at end of year	4.6	[1]	4.7
Balance, Excluding held-for-sale			4.7
Equity Securities and Other Investments [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	$ 4.7
Balance at end of year			$ 4.7

[1] There were no Level 3 held-for-sale securities (Chaucer) at December 31, 2018.