Investment Securities (Summary Of Investment Securities) (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Amortized cost	$ 6,193,594	$ 5,823,190
Credit Related To Other-Than-Temporary Impairments [Member]
Schedule of Investments [Line Items]
Amortized cost	10,500	10,500
Other-Than-Temporarily Impaired Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	$ 21,300	$ 21,600