Liability Related to Sale of Future Royalties - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Liability Related to Sale of Future Royalties [Line Items]
Proceeds from issuance of common stock				$ 0	$ 112,224
Allocated Consideration				100,000
Transaction costs incurred in connection with the Royalty Monetization and RPI Common stock				1,800
Non-cash interest expense recognized				$ 13,026	9,954
Effective tax rate				21.00%		34.00%
Liability Related to Sale of Future Royalties [Member]
Liability Related to Sale of Future Royalties [Line Items]
Allocated Consideration				$ 92,300
Common Stock [Member]
Liability Related to Sale of Future Royalties [Line Items]
Allocated Consideration				7,700
Royalty Purchase Agreement [Member]
Liability Related to Sale of Future Royalties [Line Items]
Cash payment under Royalty Agreement	$ 90,000
Common stock, shares issued in period	875,676
Proceeds from issuance of common stock	$ 10,000
Fair value for the liability related to sale of future royalties		$ 131,600		$ 131,600
Effective annual non-cash interest rate in calculating liability related to sale of future royalties		17.00%		17.00%
Common stock fair value				$ 8,100
Annual pre-tax interest rate in calculating liability related to sale of future royalties		17.00%		17.00%
Non-cash interest expense recognized		$ 4,600	$ 3,900	$ 13,000	$ 9,900
Effective tax rate				21.00%
Royalty Purchase Agreement [Member] | Scenario Previously Reported [Member]
Liability Related to Sale of Future Royalties [Line Items]
Fair value for the liability related to sale of future royalties		$ 96,700		$ 96,700