Investment Securities (Roll Forward of credit losses on securities) (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Balance of cumulative credit losses at December 31, 2011	$ 55,738
Additions for credit losses on securities for which OTTI was not previously recognized	85
Additions for additional credit losses on securities for which OTTI was previously recognized	3,034
Balance of cumulative credit losses at June 30, 2012	$ 58,857