Short -Term Investments - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Minimum
Schedule of Investments [Line Items]
Short-term investment securities, maturity date	90 days
Remaining months for contractual maturities of short-term investments, maximum	1 month	1 month
Maximum
Schedule of Investments [Line Items]
Remaining months for contractual maturities of short-term investments, maximum	6 months	7 months