Debt Securities for which Credit Loss was Recognized in Income and Other Losses Recorded in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2013	Dec. 31, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Credit losses on debt securities held, beginning of year	$ 3,567	$ 1,205	$ 1,205
Reductions related to actual losses incurred	(2,362)
Credit losses on debt securities held, end of period	$ 1,205	$ 1,205	$ 1,205