Basis of Presentation (Schedule of Effect of Retrospective Adoption on Individual Financial Statement Line Items in Consolidated Statement of Income (Loss)) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Basis Of Presentation [Line Items]
Underwriting, acquisition and insurance expenses	$ 118.0	$ 104.2	$ 346.3	$ 314.7
Loss before income taxes	17.7	(4.0)	70.8	(70.0)
Provision for income taxes	4.3	6.8	13.8	13.3
Net loss	13.4	(10.8)	57.0	(83.3)
Net loss per common share:
Basic	$ 0.53	$ (0.39)	$ 2.22	$ (3.04)
Diluted	$ 0.52	$ (0.39)	$ 2.18	$ (3.04)
As Previously Reported [Member]
Basis Of Presentation [Line Items]
Underwriting, acquisition and insurance expenses		106.3		315.9
Loss before income taxes		(6.1)		(71.2)
Provision for income taxes		6.0		12.6
Net loss		(12.1)		(83.8)
Net loss per common share:
Basic		$ (0.44)		$ (3.06)
Diluted		$ (0.44)		$ (3.06)
As Adjusted [Member]
Basis Of Presentation [Line Items]
Underwriting, acquisition and insurance expenses		104.2		314.7
Loss before income taxes		(4.0)		(70.0)
Provision for income taxes		6.8		13.3
Net loss		(10.8)		(83.3)
Net loss per common share:
Basic		$ (0.39)		$ (3.04)
Diluted		$ (0.39)		$ (3.04)
Effect of Change [Member]
Basis Of Presentation [Line Items]
Underwriting, acquisition and insurance expenses		(2.1)		(1.2)
Loss before income taxes		2.1		1.2
Provision for income taxes		0.8		0.7
Net loss		$ 1.3		$ 0.5
Net loss per common share:
Basic		$ 0.05		$ 0.02
Diluted		$ 0.05		$ 0.02