Securities Available for Sale - Roll-Forward of Credit Loss Component of Other-than-Temporary Impairment (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Beginning credit loss amount	$ 1,240	$ 938
Add: Initial OTTI credit losses
Subsequent OTTI credit losses		302
Less: Realized losses for securities sold
Securities intended or required to be sold
Increases in expected cash flows on debt securities
Ending credit loss amount	$ 1,240	$ 1,240