Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues									$ 4,822.3	$ 4,781.3	$ 4,545.0
Net realized and unrealized investment gains (losses)									5.0	109.4	(50.7)
Total revenues	$ 1,297.0	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5	4,827.3	4,890.7	4,494.3
Net investment income									265.1	281.3	267.4
Operating income (loss) before interest expense and income taxes									484.7	453.6	406.5
Interest on debt									(37.1)	(37.5)	(45.1)
Operating income before income taxes									447.6	416.1	361.4
Net loss from repayment of debt									(6.2)		(28.2)
Other non-operating items									(1.6)	(3.4)
Income from continuing operations before income taxes									444.8	522.1	282.5
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,870.8	2,843.5	2,739.5
Underwriting income (loss)									106.3	121.5	85.0
Net investment income									175.3	180.1	182.2
Other income (expense)									(6.2)	(1.5)	(1.5)
Operating income (loss) before interest expense and income taxes									275.4	300.1	265.7
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,931.4	1,911.8	1,791.3
Underwriting income (loss)									132.9	59.1	66.6
Net investment income									76.7	80.1	73.7
Other income (expense)									2.9	5.7	5.9
Operating income (loss) before interest expense and income taxes									212.5	144.9	146.2
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues									20.1	26.0	14.2
Underwriting income (loss)									(4.3)	(1.3)	(4.2)
Net investment income									13.1	21.1	11.5
Other income (expense)									(12.0)	(11.2)	(12.7)
Operating income (loss) before interest expense and income taxes									$ (3.2)	$ 8.6	$ (5.4)