Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($)	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	$ 173,097,000	$ 162,251,000	$ 0
Assets fair value disclosure	208,608,000	216,355,000
Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	27,000	39,000
Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	163,072,000	157,222,000
Commercial Paper [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	9,998,000	4,990,000
Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	35,511,000	54,104,000
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	27,000	39,000
Quoted Prices in Active Markets (Level 1) [Member] | Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	27,000	39,000
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	208,581,000	216,316,000
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	163,072,000	157,222,000
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	9,998,000	4,990,000
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	$ 35,511,000	$ 54,104,000