Financial Instruments Measured at Fair Value on Recurring Basis by Level within Fair Value Hierarchy (Detail) (USD $)	Mar. 31, 2013	Jan. 24, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities	$ 183,671,000		$ 118,187,000
Financial liability	3,861,000	3,124,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	31,063,000		28,610,000
Marketable securities	183,671,000		118,187,000
Financial Assets Total	214,734,000		146,797,000
Financial Liabilities Total	4,642,000
Fair Value, Measurements, Recurring | Warrant Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability	781,000		835,000
Fair Value, Measurements, Recurring | Derivative Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability	3,861,000
Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	4,753,000		25,781,000
Marketable securities	1,998,000		1,997,000
Financial Assets Total	6,751,000		27,778,000
Financial Liabilities Total
Level 1 | Fair Value, Measurements, Recurring | Warrant Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability
Level 1 | Fair Value, Measurements, Recurring | Derivative Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	26,310,000		2,829,000
Marketable securities	181,673,000		116,190,000
Financial Assets Total	207,983,000		119,019,000
Financial Liabilities Total
Level 2 | Fair Value, Measurements, Recurring | Warrant Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability
Level 2 | Fair Value, Measurements, Recurring | Derivative Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability
Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents
Marketable securities
Financial Assets Total
Financial Liabilities Total	4,642,000
Level 3 | Fair Value, Measurements, Recurring | Warrant Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability	781,000		835,000
Level 3 | Fair Value, Measurements, Recurring | Derivative Liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial liability	$ 3,861,000