Gross Realized Gains and Losses on Sales and Calls of Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	$ 10,532	$ 12,000	$ 13,920	$ 22,762	$ 28,693
Gross realized losses	(214)			(214)	(1,600)
Total Realized Gains on Sales and Calls of AFS Securities, net	10,318		13,920	22,881	27,206
Call Option
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains				$ 333	$ 113