Significant Accounting Policies - Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of allowance for doubtful accounts
Beginning allowance for doubtful accounts	$ 10,201	$ 10,024	$ 8,835
Additions charged to expense	21,751	16,503	13,067
Write-offs, net of recoveries	(19,265)	(16,326)	(11,878)
Ending allowance for doubtful accounts	$ 12,687	$ 10,201	$ 10,024