Schedule II - Valuation and Qualifying Accounts - Allowance for Doubtful Accounts (Details) - Allowance for doubtful accounts deducted from accounts receivable in the balance sheet - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period		$ 2,370	$ 1,216	$ 1,253
Additions charged to cost and expenses	[1]	1,598	497	674
Deductions	[2]	(1,314)	657	(711)
Balance at end of period		$ 2,654	$ 2,370	$ 1,216

[1] Adjustments to allowance for change in estimates.

[2] Uncollectible accounts written off, net of recoveries.