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
Movement in Valuation Allowances and Reserves
Balance at Beginning of Year	$ 1,501	$ 1,473	$ 918
Additions: Charged Against Operations	4,459	53	557
Deductions: Uncollectible Accounts Written Off	(5,289)	(25)	(2)
Balance at End of Year	$ 671	$ 1,501	$ 1,473