Segment Information (Revenue And Income (Loss) Before Income Taxes For Each Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Revenue And Loss Income Before Income Taxes [Line Items]
Earned premiums	$ 304.3	$ 270.9	$ 871.8	$ 804.0
Earned premiums	304.3	270.9	871.8	804.0
Net investment income	28.9	30.0	90.3	96.3
Fee income, net	3.0	1.4	4.8	1.8
Net realized investment (losses) gains	8.3	3.9	18.7	37.7
Total revenue	344.5	306.2	985.6	939.8
Total segment income (loss) before taxes	28.3	(3.7)	83.0	(80.6)
Net realized investment (losses) gains	8.3	3.9	18.7	37.7
Total income (loss) before income taxes	17.7	(4.0)	70.8	(70.0)
Excess and Surplus Lines
Revenue And Loss Income Before Income Taxes [Line Items]
Earned premiums	100.8	99.5	295.5	305.8
Earned premiums	100.8	99.5	295.5	305.8
Net investment income	12.5	13.4	38.5	42.8
Total segment income (loss) before taxes	15.1	21.1	56.3	41.6
Commercial Specialty
Revenue And Loss Income Before Income Taxes [Line Items]
Earned premiums	78.6	82.9	241.8	233.7
Earned premiums	78.6	82.9	241.8	233.7
Net investment income	6.7	6.6	20.6	21.0
Total segment income (loss) before taxes	(2.7)	(1.2)	(11.8)	(7.3)
International Specialty
Revenue And Loss Income Before Income Taxes [Line Items]
Earned premiums	33.4	21.3	90.8	76.0
Earned premiums	33.4	21.3	90.8	76.0
Net investment income	3.1	2.4	10.0	8.1
Total segment income (loss) before taxes	6.6	(13.1)	22.0	(63.9)
Syndicate 1200 [Member]
Revenue And Loss Income Before Income Taxes [Line Items]
Earned premiums	90.8	67.3	242.3	188.2
Earned premiums	90.8	67.3	242.3	188.2
Net investment income	3.8	4.3	11.4	13.3
Total segment income (loss) before taxes	11.8	(4.7)	18.0	(49.0)
Run-Off Lines
Revenue And Loss Income Before Income Taxes [Line Items]
Earned premiums	0.7	(0.1)	1.4	0.3
Earned premiums	0.7	(0.1)	1.4	0.3
Net investment income	2.8	3.1	9.8	10.4
Total segment income (loss) before taxes	(2.5)	(5.8)	(1.5)	(2.0)
Corporate and Other
Revenue And Loss Income Before Income Taxes [Line Items]
Net investment income	0	0.2	0	0.7
Net realized investment (losses) gains	(18.9)	(4.2)	(30.9)	(27.1)
Net realized investment (losses) gains	$ (18.9)	$ (4.2)	$ (30.9)	$ (27.1)