Significant Accounting Policies - Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for doubtful accounts, beginning of period	$ 30,733	$ 15,048	$ 28,457	$ 12,687
Additions charged to expense	12,111	9,864	35,893	22,851
Adjustment to value of acquired receivables	0	6,601	2,207	6,601
Write-offs, net of recoveries	(11,314)	(6,118)	(35,027)	(16,744)
Allowance for doubtful accounts, end of period	$ 31,530	$ 25,395	$ 31,530	$ 25,395