Summary of Significant Accounting Policies - Letter of Credit Facility (Details) - Wells Fargo Bank [Member]	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 60,000,000
Current ratio	1.1
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letters of credit outstanding	$ 2,200,000	$ 0