Significant Accounting Policies - Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of allowance for doubtful accounts
Beginning allowance for doubtful accounts	$ 10,201	$ 10,024
Additions charged to expense	4,382	3,068
Write-offs, net of recoveries	(3,763)	(2,866)
Ending allowance for doubtful accounts	$ 10,820	$ 10,226