Securities - Net Gains and Losses on Trading Account Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Investments Debt And Equity Securities [Abstract]
Net gain on sales transactions	$ 282	$ 299	$ 576	$ 622
Net mark-to-market gains (losses)	(377)	(19)	(380)	(79)
Net gain (loss) on trading account securities	$ (95)	$ 280	$ 196	$ 543