Segment Information (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Number of operating segments of the Company			three
P And C Segment [Member]
Segment Reporting Information Structured Products, Interest Income	$ 18.9	$ 15.3	$ 57.0	$ 52.2
Segment Reporting Information Structured Products, Interest Expense	13.1	22.1	38.8	43.6
Segment Reporting Information Structured Products Operating Income Expenses				1.3
Corporate [Member]
Segment Reporting Information Structured Products, Interest Income		2.0		11.0
Segment Reporting Information Structured Products, Interest Expense		0.3		1.7
Segment Reporting Information Structured Products Operating Expenses		$ 0.1		$ 1.0