Fair Value of Financial Instruments and Concentrations of Credit Risk - Allowances for Losses on Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 9,457	$ 8,849	$ 7,189
Provision for uncollectible accounts	2,849	1,481	2,416
Write-offs, net of recoveries	(2,272)	(873)	(756)
Balance at end of year	$ 10,034	$ 9,457	$ 8,849