Marketable Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended				12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Government and agency securities Level 1	Sep. 30, 2011 Government and agency securities Level 1	Dec. 31, 2011 Government and agency securities Level 1
Schedule of Available-for-sale Securities [Line Items]
Transfers between levels during the period	0	0	0	0
Available for sale securities, maturity period					12 months	12 months	12 months