Line of Credit - Additional Information (Detail) (USD $)	9 Months Ended								1 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 19, 2013	Aug. 31, 2013	Dec. 31, 2012	Sep. 30, 2013 Standby Letter of Credit	May 31, 2013 Standby Letter of Credit Previous Credit Facility	May 31, 2013 Standby Letter of Credit New Credit Facility	Sep. 19, 2013 Amended and Restated Credit Agreement	Sep. 30, 2013 Amended and Restated Credit Agreement	Sep. 30, 2013 Silicon Valley Bank	Aug. 31, 2013 Line of Credit Amended Available Borrowing
Line of Credit Facility [Line Items]
Outstanding letter of credit						$ 15,000,000	$ 10,000,000	$ 15,000,000
Line of credit	150,000,000			80,000,000					150,000,000			100,000,000
Line of credit, expiration date									Sep. 19, 2016
Portion of line of credit classified as non-current liability			25,000,000		25,000,000
Loan agreement, interest rate at period end	4.25%				4.50%
Line of credit, percentage of annual fee										0.50%
Commitment fee										430,000
Line of credit facility, one-time arrangement fee percentage											0.30%
Line of credit facility, annual administration fee paid											45,000
Borrowings under credit facility	532,000,000	437,500,000
Repayments of credit facility	572,000,000	423,500,000
Available credit facility	$ 135,000,000