Fair Value Measurements (Financial Instruments Measured at Fair Value on a Recurring Basis) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018		Jan. 06, 2018	Dec. 31, 2017
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Royalty rights	$ 341,691	[1]		$ 349,223	[2]
Financial assets:
Assets, Fair Value	664,576			771,663
Warrants and Rights Outstanding	100			29
Financial liabilites:
Business Combination, Contingent Consideration, Liability, Noncurrent	41,400	[1]	$ 640	42,000	[1]
Liabilities, Fair Value Disclosure, Recurring	41,400			42,000
Money Market Funds [Member]
Financial assets:
Cash and Cash Equivalents, Fair Value	322,785			417,563
Equity Securities [Member]
Financial assets:
Available for Sale Securities, Fair Value	0			4,848
Fair Value Level 1 [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0			0
Financial assets:
Assets, Fair Value	322,785			422,411
Fair Value Level 1 [Member] | Money Market Funds [Member]
Financial assets:
Cash and Cash Equivalents, Fair Value	322,785			417,563
Fair Value Level 1 [Member] | Equity Securities [Member]
Financial assets:
Available for Sale Securities, Fair Value	0			4,848
Fair Value Level 2 [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0			0
Financial assets:
Assets, Fair Value	100			29
Warrants and Rights Outstanding	100			29
Fair Value Level 2 [Member] | Money Market Funds [Member]
Financial assets:
Cash and Cash Equivalents, Fair Value	0			0
Fair Value Level 2 [Member] | Equity Securities [Member]
Financial assets:
Available for Sale Securities, Fair Value	0			0
Fair Value Level 3 [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	41,400			42,000
Royalty rights	341,691			349,223
Financial assets:
Assets, Fair Value	341,691			349,223
Warrants and Rights Outstanding	0
Financial liabilites:
Business Combination, Contingent Consideration, Liability, Noncurrent	$ 41,400			$ 42,000

[1] unaudited

[2] Note 1