Schedule II-Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in valuation and qualifying accounts
Balance at beginning of period	$ 1,433	$ 475	$ 1,149
Additions:
Bad debt expense	1,044	2,895	(277)
Deductions:
Charge-offs of uncollectible balances	(502)	(1,937)	(397)
Balance at end of period	$ 1,975	$ 1,433	$ 475