Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 1,354.3	$ 1,259.0
Net realized and unrealized investment (losses) gains	(23.4)	1.9
Total revenues	1,330.9	1,260.9
Net investment income	82.9	71.1
Operating income (loss) before interest expense and income taxes	116.6	69.1
Interest on debt	(12.4)	(12.0)
Operating income before income taxes	104.2	57.1
Other non-operating items	(0.2)
Income before income taxes	80.6	59.0
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	676.6	630.5
Underwriting income (loss)	16.4	(2.7)
Net investment income	45.3	40.3
Other income (expense)	(0.2)	(0.2)
Operating income (loss) before interest expense and income taxes	61.5	37.4
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	434.9	401.8
Underwriting income (loss)	14.7	(8.4)
Net investment income	18.4	17.1
Other income (expense)	1.3	1.2
Operating income (loss) before interest expense and income taxes	34.4	9.9
Chaucer [Member]
Segment Reporting Information [Line Items]
Operating revenues	239.4	224.7
Underwriting income (loss)	6.2	13.8
Net investment income	16.4	12.4
Other income (expense)	0.3	(1.3)
Operating income (loss) before interest expense and income taxes	22.9	24.9
Other (including eliminations) [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.4	2.0
Underwriting income (loss)	(1.3)	(0.9)
Net investment income	2.8	1.3
Other expense	(3.7)	(3.5)
Operating income (loss) before interest expense and income taxes	$ (2.2)	$ (3.1)