Significant Accounting Policies - Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of allowance for doubtful accounts
Beginning allowance for doubtful accounts	$ 10,820	$ 10,226	$ 10,201	$ 10,024
Additions charged to expense	5,090	4,136	9,472	7,204
Write-offs, net of recoveries	(4,150)	(4,381)	(7,913)	(7,247)
Ending allowance for doubtful accounts	$ 11,760	$ 9,981	$ 11,760	$ 9,981