Valuation and Qualifying Accounts (Details) - Allowance for doubtful receivables [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 2,974	$ 2,927	$ 1,934
Charged to costs and expenses	4,047	1,994	2,900
Write-offs	(2,710)	(1,947)	(1,145)
Transfers, net			(762)
Balance at end of year	$ 4,311	$ 2,974	$ 2,927