Securities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 Security	Dec. 31, 2011 Security
Schedule of Available-for-sale Securities [Line Items]
Carrying value of securities	$ 174,773	$ 137,388
Unrealized loss position on securities, current	Less than 12 months
Unrealized loss position on securities, non current	More than 12 months
Number of securities	10	10
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	$ (244)	$ (337)