Summary Of Cumulative Amounts Related To Credit Loss Portion Of OTTI Losses On Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 3,321	$ 2,228
Additions for credit loss impairments recognized in the current period on securities not previously impaired
Additions for credit loss impairments recognized in the current period on securities previously impaired	11	11
Reductions for credit loss impairments previously recognized on securities sold during the period
Ending balance	$ 3,332	$ 2,239