Credit Quality Distribution of Reinsurance Recoverables for Ceded Paid and Unpaid Losses and Loss Adjustment Expenses and Ceded Unearned Premiums Based On Insurer Financial Strength Ratings from A. M. Best or S&P (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Minimum	Dec. 31, 2012 Maximum	Dec. 31, 2012 Very Good	Dec. 31, 2012 Not rated
Ceded Credit Risk [Line Items]
Reserve for uncollectible reinsurance amount	$ 11.5	$ 13.0
Reinsurance recoverable due date			30 days	45 days
Percentage of offsetting collateral held	19.70%				17.60%	54.40%