Significant Accounting Policies - Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of allowance for doubtful accounts
Beginning allowance for doubtful accounts	$ 11,760	$ 9,981	$ 10,201	$ 10,024
Additions charged to expense	5,364	3,865	14,835	11,069
Write-offs, net of recoveries	(4,567)	(4,052)	(12,479)	(11,299)
Ending allowance for doubtful accounts	$ 12,557	$ 9,794	$ 12,557	$ 9,794