Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 31.0	$ 39.6
Transfers out of Level 3	(3.5)
Included in total net realized and unrealized investment gains	3.9	1.4
Included in other comprehensive income - net appreciation on available-for-sale securities	0.8
Purchases and Sales:
Purchases		1.0
Sales	(2.8)	(11.0)
Ending Balance	29.4	31.0
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	1.0
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	12.1	21.1
Transfers out of Level 3	(3.5)
Included in total net realized and unrealized investment gains		0.9
Included in other comprehensive income - net appreciation on available-for-sale securities	0.1
Purchases and Sales:
Sales	(1.7)	(9.9)
Ending Balance	7.0	12.1
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	0.3
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.6	0.8
Purchases and Sales:
Sales	(0.1)	(0.2)
Ending Balance	0.5	0.6
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	12.7	13.1
Included in total net realized and unrealized investment gains		0.5
Included in other comprehensive income - net appreciation on available-for-sale securities	0.7
Purchases and Sales:
Sales	(1.0)	(0.9)
Ending Balance	12.4	12.7
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	0.7
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	25.4	35.0
Transfers out of Level 3	(3.5)
Included in total net realized and unrealized investment gains		1.4
Included in other comprehensive income - net appreciation on available-for-sale securities	0.8
Purchases and Sales:
Sales	(2.8)	(11.0)
Ending Balance	19.9	25.4
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	1.0
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	5.6	4.6
Included in total net realized and unrealized investment gains	3.9
Purchases and Sales:
Purchases		1.0
Ending Balance	$ 9.5	$ 5.6