Investment Securities (Detail) - Cumulative credit-related losses recognized in earnings (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Cumulative credit related losses recognized in earnings at beginning of period	$ 8,605	$ 8,042	$ 8,391	$ 7,866
Cumulative credit related losses recognized in earnings at end of period	8,750	8,211	8,750	8,211
Additions
Credit loss for which previous OTTI was recognized	145	169	359	345
Collateralized Debt Obligations [Member]
Cumulative credit related losses recognized in earnings at beginning of period	5,823	5,587	5,782	5,411
Cumulative credit related losses recognized in earnings at end of period	5,823	5,756	5,823	5,756
Additions
Credit loss for which previous OTTI was recognized	0	169	41	345
Collateralized Mortgage Obligations [Member]
Cumulative credit related losses recognized in earnings at beginning of period	1,682	1,355	1,509	1,355
Cumulative credit related losses recognized in earnings at end of period	1,827	1,355	1,827	1,355
Additions
Credit loss for which previous OTTI was recognized	145	0	318	0
Equity Securities, Other [Member]
Cumulative credit related losses recognized in earnings at beginning of period	1,100	1,100	1,100	1,100
Cumulative credit related losses recognized in earnings at end of period	1,100	1,100	1,100	1,100
Additions
Credit loss for which previous OTTI was recognized	$ 0	$ 0	$ 0	$ 0