Segment Information (Financial Information with Respect to Business Segments) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 1,250.8	$ 1,168.9	$ 2,490.3	$ 2,341.1
Net realized investment gains	22.2	13.7	26.6	21.8
Net investment income	67.0	67.9	134.0	135.2
Total revenues	1,273.0	1,182.6	2,516.9	2,362.9
Operating income (loss) before interest expense and income taxes	102.2	81.1	187.2	183.3
Interest on debt	(16.3)	(17.4)	(32.6)	(32.1)
Operating income before income taxes	85.9	63.7	154.6	151.2
Net loss from repayment of debt		(11.3)		(19.1)
Net benefit (costs) related to acquired businesses	(0.2)	(0.2)	(1.1)	0.2
Net foreign exchange gains (losses)	(0.2)		(0.3)	0.7
Income before income taxes	107.7	65.9	179.8	154.8
Commercial Lines
Segment Reporting Information [Line Items]
Operating revenues	556.9	523.7	1,108.9	1,038.4
GAAP underwriting income (loss)	7.3	(9.5)	(14.0)	(12.6)
Net investment income	37.2	36.0	74.5	72.1
Other income (expense)	(0.3)	(0.3)	(0.2)	(0.3)
Operating income (loss) before interest expense and income taxes	44.2	26.2	60.3	59.2
Personal Lines
Segment Reporting Information [Line Items]
Operating revenues	371.4	388.9	742.4	780.0
GAAP underwriting income (loss)	3.6	(0.3)	5.7	9.8
Net realized investment gains	18.0	19.1	35.9	38.1
Other income (expense)	1.1	1.1	2.7	2.5
Operating income (loss) before interest expense and income taxes	22.7	19.9	44.3	50.4
Chaucer
Segment Reporting Information [Line Items]
Operating revenues	320.6	253.5	635.1	517.4
GAAP underwriting income (loss)	24.3	24.7	60.8	57.7
Net investment income	10.7	10.7	21.2	21.2
Other income (expense)	3.1	1.5	5.3	(1.1)
Operating income (loss) before interest expense and income taxes	38.1	36.9	87.3	77.8
Other
Segment Reporting Information [Line Items]
Operating revenues	1.9	2.8	3.9	5.3
GAAP underwriting income (loss)	(0.9)	(1.0)	(1.4)	(2.1)
Net investment income	1.1	2.1	2.4	3.8
Other net expenses	(3.0)	(3.0)	(5.7)	(5.8)
Operating income (loss) before interest expense and income taxes	(2.8)	(1.9)	(4.7)	(4.1)
Interest on debt	$ (16.3)	$ (17.4)	$ (32.6)	$ (32.1)