Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 1,359.1	$ 1,260.2	$ 2,713.4	$ 2,519.2
Net realized and unrealized investment gains (losses)	3.7	5.9	(19.7)	7.8
Total revenues	1,362.8	1,266.1	2,693.7	2,527.0
Net investment income	78.7	72.3	161.6	143.4
Operating income (loss) before interest expense and income taxes	129.8	118.9	246.4	188.0
Interest on debt	(11.6)	(12.2)	(24.0)	(24.2)
Operating income before income taxes	118.2	106.7	222.4	163.8
Other non-operating items	(2.0)	(1.6)	(2.2)	(1.6)
Income before income taxes	119.9	111.0	200.5	170.0
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	681.7	634.5	1,358.3	1,265.0
Underwriting income (loss)	37.7	2.3	54.1	(0.4)
Net investment income	44.8	41.3	90.1	81.6
Other income (expense)	(0.5)	(0.4)	(0.7)	(0.6)
Operating income (loss) before interest expense and income taxes	82.0	43.2	143.5	80.6
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	444.5	411.5	879.4	813.3
Underwriting income (loss)	8.0	29.5	22.7	21.1
Net investment income	18.1	17.4	36.5	34.5
Other income (expense)	1.2	1.0	2.5	2.2
Operating income (loss) before interest expense and income taxes	27.3	47.9	61.7	57.8
Chaucer [Member]
Segment Reporting Information [Line Items]
Operating revenues	228.8	211.2	468.2	435.9
Underwriting income (loss)	9.3	18.0	15.5	31.8
Net investment income	12.5	11.3	28.9	23.7
Other income (expense)	0.1	0.4	0.4	(0.9)
Operating income (loss) before interest expense and income taxes	21.9	29.7	44.8	54.6
Other (including eliminations) [Member]
Segment Reporting Information [Line Items]
Operating revenues	4.1	3.0	7.5	5.0
Underwriting income (loss)	(1.4)	(0.9)	(2.7)	(1.8)
Net investment income	3.3	2.3	6.1	3.6
Other expense	(3.3)	(3.3)	(7.0)	(6.8)
Operating income (loss) before interest expense and income taxes	$ (1.4)	$ (1.9)	$ (3.6)	$ 5.0