Build-to-Suit Lease (Details) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2016 ft²	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Apr. 30, 2016 USD ($)
Build-to-Suit Lease Obligation
Built-to-suit leases, at cost		$ 52,000	$ 52,000
Build-to-suit lease obligation, current and noncurrent		51,913	51,913
Build-to-suit lease obligation, non-current		50,338	50,338		$ 51,515
Build-to-suit lease obligation, current		1,575	$ 1,575		$ 733
Built-to-Suit Lease [Member]
Build-to-Suit Lease Obligation
Build-to-suit Area of real estate property (in sq ft) | ft²	128,751
Build-to-suit lease expiration date	Dec. 31, 2023
Build-to-suit lease, term of contract			7 years 9 months 30 days
Total lease payments due over lease term						$ 39,200
Tenant improvement allowance				$ 14,200
Amount capitalized under Build-to-Suit Transaction			$ 36,500
Interest capitalized during construction period for build-to-suit lease transaction			1,200
Tenant improvements		15,500	15,500
Proceeds from tenant improvement allowance under build-to-suit lease transaction				$ 14,200
Ground rent		100	400
Line of credit facility						$ 4,100
Line of credit has been used		0	0
Other Nonoperating Income (Expense) [Member] | Built-to-Suit Lease [Member]
Build-to-Suit Lease Obligation
Build-to-suit lease, interest expense		$ 900	$ 2,800