Long-term Obligations - San Diego Lease (Details) ft² in Thousands, $ in Thousands	Dec. 02, 2015 USD ($) ft² term_extension	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Property and equipment, net		$ 800,670	$ 789,437
San Diego Lease
Property, Plant and Equipment [Line Items]
Area of real estate property (in square feet) | ft²	170
Length of lease	16 years
Average yearly aggregate rent	$ 10,200
Amount of optional renewal terms | term_extension	2
Optional renewal term length	5 years
San Diego Lease | Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, net		105,700	94,600
San Diego Lease | Buildings
Property, Plant and Equipment [Line Items]
Construction financing lease obligation, current and noncurrent		$ 91,100	$ 87,400