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Asset Acquisition and License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended		12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2004	Dec. 31, 2017	Dec. 31, 2010
Asset Acquisition [Line Items]
Restricted cash (SRX Cardio)		$ 178,000			$ 47,000	$ 174,000		$ 173,000
Net loss attributable to noncontrolling interest (SRX Cardio)					332,000	45,000
Research and development expense					60,067,000	30,645,000
Millennium Pharmaceuticals, Inc
Asset Acquisition [Line Items]
Notice period for agreement termination							30 days
Millennium Pharmaceuticals, Inc \| United States
Asset Acquisition [Line Items]
Milestone payment on regulatory approval	$ 5,000,000
Additional milestone payable for other indications approved					23,000,000
Millennium Pharmaceuticals, Inc \| Europe
Asset Acquisition [Line Items]
Milestone payable for approval event					5,000,000
Additional milestone payable for other indications approved					23,000,000
Millennium Pharmaceuticals, Inc \| Research and Development Expense
Asset Acquisition [Line Items]
Milestone payment		$ 2,000,000
Millennium Pharmaceuticals, Inc \| Cost of Sales \| United States
Asset Acquisition [Line Items]
Finite-lived intangible assets, amortization expenses					141,000
Astellas Pharma Inc
Asset Acquisition [Line Items]
Percentage required to pay for any payment received under collaboration excluding royalties									20.00%
Research and development expense					$ 0	$ 0
Maximum \| Millennium Pharmaceuticals, Inc
Asset Acquisition [Line Items]
Percentage of royalty on product sales					8.00%
Maximum \| Astellas Pharma Inc
Asset Acquisition [Line Items]
Milestone payable on occurrence of specified events									$ 71,500,000
Minimum \| Millennium Pharmaceuticals, Inc
Asset Acquisition [Line Items]
Percentage of royalty on product sales					2.00%
SRX Cardio
Asset Acquisition [Line Items]
Variable interest entity, Upfront payment			$ 2,200,000	$ 500,000
Variable interest entity, Terms of arrangements					Accordingly, SRX Cardio is subject to consolidation and we have consolidated the financial statements of SRX Cardio by (a) eliminating all intercompany balances and transactions; and (b) allocating income or loss attributable to the noncontrolling interest in SRX Cardio to net income or loss attributable to noncontrolling interest in our consolidated statement of operations and reflecting noncontrolling interest on our consolidated balance sheet.
Fair value of indefinite lived intangible assets					$ 3,200,000
Noncontrolling interest in variable Interest entity					2,900,000
Restricted cash (SRX Cardio)					$ 47,000			$ 173,000
SRX Cardio \| License Agreement
Asset Acquisition [Line Items]
Notice period for agreement termination			90 days
SRX Cardio \| License Agreement \| Maximum
Asset Acquisition [Line Items]
Research and development milestone payment obligation			$ 152,500,000
Percentage of royalties on worldwide net sales, obligated to pay			6.00%
SRX Cardio \| License Agreement \| Minimum
Asset Acquisition [Line Items]
Percentage of royalties on worldwide net sales, obligated to pay			2.00%