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Nature of Business and Basis of Presentation - Additional Information (Detail) (USD $)	6 Months Ended				1 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011	Jul. 12, 2013 Subsequent Event
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Cash and cash equivalents	$ 149,426,000	$ 260,242,000	$ 250,255,000	$ 116,602,000
Net proceeds from public offering					$ 121,100,000
Convertible senior notes, earliest redemption date	Jan. 19, 2017

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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