Mortgage Backed Security Credit Losses Recognized In Earnings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
TemporaryImpairmentCreditLossesRecognizedInEarningsRollForward
Balance, beginning of year	$ 3,455	$ 9,757	$ 17,266
Reversal of interest reserve		(169)
Realized pass through of actual losses	(1,313)	(6,412)	(7,730)
Amounts related to credit loss for which an other-than- temporary impairment was not previously recognized		279	221
Balance, end of year	$ 2,142	$ 3,455	$ 9,757