Summary of Cumulative Amounts Related to Credit Loss Portion of the OTTI Losses on Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 2,228	$ 2,523
Additions for credit loss impairments recognized in the current period on securities not previously impaired	109	271	3,102
Additions for credit loss impairments recognized in the current period on securities previously impaired	984	422
Reductions for credit loss impairments previously recognized on securities sold during the period		(988)	(579)
Ending balance	$ 3,321	$ 2,228	$ 2,523