Leases - Additional Information (Details)			1 Months Ended			12 Months Ended			22 Months Ended
	Mar. 27, 2017	Sep. 21, 2015 USD ($) ft² $ / ft²	Sep. 30, 2019 ft² lease_term $ / ft²	Apr. 30, 2019 USD ($) ft² $ / ft²	Jul. 31, 2018 USD ($) ft² $ / ft²	Dec. 31, 2019 USD ($) lease_term	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Jun. 03, 2016
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets						$ 185,885,000
Total operating lease liabilities						190,987,000
lease arrangements annual rent increase percentage				3.00%
Operating lease, expense						45,300,000
Rent expense							$ 9,800,000	$ 9,000,000.0
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets										$ 184,400,000
Total operating lease liabilities										177,000,000.0
Seattle, Washington
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) | ft²			22,188		36,126
Annual lease rent per square foot (in dollars per sq ft) | $ / ft²			62.80		54.00
Lease rent, annual increase percentage			2.50%		2.50%
Lease extension terms			5 years
Tenant improvement allowance per square foot					$ 215.00
Tenant improvement allowance					$ 8,000,000.0
Proceeds from tenant allowance						$ 8,000,000.0
Lessee, operating lease, term of contract extension			16 months
Lessee, operating lease, number of contracts | lease_term			2
60 Binney Street lease
Lessee, Lease, Description [Line Items]
Lessee, operating lease, number of terms | lease_term						2
Lease extension terms						5 years
Estimated useful lives of assets	40 years
60 Binney Street lease | Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets										127,300,000
Total operating lease liabilities										$ 125,800,000
50 Binney Street sublease
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) | ft²				267,278
Collateralized letter of credit				$ 40,100,000
Rate of area for sublease per square feet (in dollars per sq ft) | $ / ft²				99.95
Lessee, operating lease, annual expense				$ 26,700,000
Lessee, operating sublease, payment commencement, term				90 days
Upfront payment for purchase of furniture and equipment				$ 7,500,000
50 Binney Street sublease | Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating sublease, option to postpone commencement, term				9 months
Manufacturing Facility
Lessee, Lease, Description [Line Items]
Lessee, operating lease, annual expense						$ 5,100,000
Lease period											12 years
Milestone paid									$ 12,000,000.0
Lessee, operating lease, lease not yet commenced, amount						$ 8,000,000.0
Lessee, operating lease, termination fees, term						24 months
Lease starting on October 1, 2016 | 60 Binney Street lease
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) | ft²		253,108
Annual lease rent per square foot (in dollars per sq ft) | $ / ft²		72.50
Lease payments base annual rent		$ 18,400,000
Lease rent, annual increase percentage		1.75%
Collateralized letter of credit		$ 13,800,000
Decrease In letter of credit under the terms of the lease		9,200,000
Landlord contribution for cost of construction and tenant improvements		$ 42,400,000