Summary of Cumulative Amounts Related to Credit Loss Portion of Other-Than-Temporary Impairment Losses on Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 3,321	$ 2,228	$ 2,523
Additions for credit loss impairments recognized in the current period on securities not previously impaired		109	271
Additions for credit loss impairments recognized in the current period on securities previously impaired	11	984	422
Reductions for credit loss impairments previously recognized on securities sold during the period			(988)
Ending balance	$ 3,332	$ 3,321	$ 2,228