Summary of Significant Accounting Policies - Letters of Credit Facility (Details) - Wells Fargo Bank - Standby Letter of Credit - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Letter of Credit Facility [Abstract]
Line of credit facility, maximum borrowing capacity	$ 60,000,000.0
Stand-by letters of credit	$ 400,000	$ 2,200,000

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant