LOSSES AND LOSS EXPENSES (Detail) - Net (favorable) adverse prior year development by line of busines relating to the Reinsurance segment (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Amount	$ (284.9)	$ (372.9)	$ (284.7)
Reinsurance Segment [Member] | Property And Other Short-tail Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Amount	(64.3)	(145.8)	(142.5)
Reinsurance Segment [Member] | Casualty and Other Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Amount	(144.1)	(99.7)	(79.3)
Reinsurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Amount	$ (208.4)	$ (245.5)	$ (221.8)