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, 2011
Ceded Credit Risk [Line Items]
Reserve for uncollectible reinsurance amount	13.0
Percentage of offsetting collateral held	16.80%
Minimum
Ceded Credit Risk [Line Items]
Reinsurance recoverable due date	30 days
Maximum
Ceded Credit Risk [Line Items]
Reinsurance recoverable due date	45 days
Very Good
Ceded Credit Risk [Line Items]
Percentage of offsetting collateral held	50.50%
Not rated
Ceded Credit Risk [Line Items]
Percentage of offsetting collateral held	62.50%