Significant Accounting Policies - Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of allowance for doubtful accounts
Beginning allowance for doubtful accounts	$ 12,687	$ 10,201
Additions charged to expense	6,391	4,382
Write-offs, net of recoveries	(5,303)	(3,763)
Ending allowance for doubtful accounts	$ 13,775	$ 10,820