Commitments and Contingencies - Textual (Details)		1 Months Ended			3 Months Ended		12 Months Ended	24 Months Ended
	Jun. 30, 2017 USD ($) multiplier	Apr. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Dec. 31, 2016	Mar. 31, 2019 USD ($) ft² building	Mar. 31, 2018 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2012 USD ($)	Feb. 28, 2019	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Commitments and Contingencies [Line Items]
Lease area space occupancy (square feet) | ft²					107,200
Number of buildings leased | building					4
Accretion expense related to asset retirement obligation					$ 0	$ 0
Incentive from lessor											$ 500,000
Asset retirement obligations					200,000						200,000
Rent expense					1,200,000	800,000
Operating lease, sublease rentals					200,000	$ 300,000
Capital leases, term of contract		3 years	3 years	3 years
Capital lease obligations incurred		$ 300,000	$ 400,000
Right-of-use assets, operating leases, net					25,913,000						0
Operating lease, liability					27,224,000						0
Right-of-use assets - finance leases, net					438,000					$ 500,000	0
Finance lease, liability					$ 242,000					300,000	0
Appeal term					90 days
Indemnification Agreement [Member]
Commitments and Contingencies [Line Items]
Loss contingency accrual					$ 0						0
Term Loan [Member]
Commitments and Contingencies [Line Items]
Borrowing capacity	$ 10,000,000
Term Loan [Member] | LIBOR [Member]
Commitments and Contingencies [Line Items]
Basis spread on variable rate	3.60%
Revolving Credit Facility [Member]
Commitments and Contingencies [Line Items]
Borrowing capacity	$ 5,000,000
Accounts receivable borrowing base percentage	80.00%
Stated interest rate	5.00%
Minimum cash multiplier | multiplier	6
Revolving Credit Facility [Member] | Prime Rate [Member]
Commitments and Contingencies [Line Items]
Basis spread on variable rate	1.00%
Accounting Standards Update 2016-02 [Member]
Commitments and Contingencies [Line Items]
Right-of-use assets, operating leases, net										26,617,000
Operating lease, liability										27,562,000
Right-of-use assets - finance leases, net										493,000
Finance lease, liability										$ 302,000
Demand Deposits [Member]
Commitments and Contingencies [Line Items]
Non-current restricted cash					$ 1,100,000						$ 700,000
Eighth Lease Amendment [Member]
Commitments and Contingencies [Line Items]
Renewal term									88 months
Payments for capital improvements							$ 3,600,000
Tenant reimbursements								$ 3,100,000
Redwood City, California, Penobscot Space [Member]
Commitments and Contingencies [Line Items]
Lease area space occupancy (square feet) | ft²					28,200
Redwood City, California, Building 2 Space [Member]
Commitments and Contingencies [Line Items]
Lease area space occupancy (square feet) | ft²					37,900
Redwood City, California, 501 Chesapeake Space [Member]
Commitments and Contingencies [Line Items]
Lease area space occupancy (square feet) | ft²					11,200
Redwood City, California, Saginaw Space [Member]
Commitments and Contingencies [Line Items]
Lease area space occupancy (square feet) | ft²					29,900