Schedule II - Valuation and Qualifying Accounts - Allowance for Credit Losses (Details) - Allowance for credit losses deducted from financing receivables 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		$ 2,198	$ 654	$ 1,001
Additions charged to cost and expenses	[1]	1,823	1,762	236
Deductions	[2]	(777)	(218)	(583)
Balance at end of period		$ 3,244	$ 2,198	$ 654

[1] Adjustments to allowance for change in estimates.

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