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, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Provision for uncollectible accounts	$ 1,481	$ 2,416	$ 1,886
Continuing operations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	8,849	7,189	5,458
Provision for uncollectible accounts	1,481	2,416	1,886
Write-offs, net of recoveries	(873)	(756)	(155)
Balance at end of year	$ 9,457	$ 8,849	$ 7,189