Accounts Receivable Securitization (Details) - Line of Credit - Accounts Receivable Securitization Facility - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 100,000,000
Unused commitment fee (as a percent)	0.60%
Accounts receivable eligible for securitization	$ 37,869,000	$ 60,582,000
Outstanding borrowings	0	0
Letters of credit outstanding	52,536,000	60,582,000
Borrowings and issuance of letters of credit remaining capacity	0
Cash collateral for letters of credit outstanding	14,667,000
Costs associated with receivables facility	$ 2,593,000	$ 171,000
Minimum
Short-term Debt [Line Items]
Security facility program fee (as a percent)	2.00%
Maximum
Short-term Debt [Line Items]
Security facility program fee (as a percent)	2.50%