Accounts Receivable Securitization (Details) - Line of Credit - Accounts Receivable Securitization Facility	12 Months Ended
	Dec. 31, 2017 USD ($)
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 100,000,000
Security facility program fee (as a percent)	4.00%
Unused commitment fee (as a percent)	0.60%
Accounts receivable eligible for securitization	$ 60,582,000
Outstanding borrowings	0
Letters of credit outstanding	60,685,000
Borrowings and issuance of letters of credit remaining capacity	0
Costs associated with receivables facility	$ 171,000