Leases - Narative (Details) ft² in Thousands, $ in Millions		3 Months Ended	12 Months Ended
	Dec. 02, 2015 ft² term_extension	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2011 ft² lease building	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Sublease income related to facility leases			$ 6.2
Rent expense		$ 4.6
Total capital leased assets, gross			94.8
Capital leases accumulated depreciation			$ 34.0
Fan Pier Leases
Property, Plant and Equipment [Line Items]
Number of leases | lease				2
Area of real estate property (in square feet) | ft²				1,100
Number of buildings under lease agreement | building				2
Length of lease				15 years
San Diego Lease
Property, Plant and Equipment [Line Items]
Area of real estate property (in square feet) | ft²	170
Length of lease					16 years
Amount of optional renewal terms | term_extension	2
Optional renewal term length	5 years