Schedule II Valuation and Qualifying Accounts (Details) - Allowance for Trade Receivables [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period		$ 4,836	$ 3,871	$ 3,838
Additions Charged to Costs and Expenses		1,276	1,747	1,459
Additions Recorded During Acquisitions		0	0	0
Deductions	[1]	112	782	1,426
Balance at End of Period		$ 6,000	$ 4,836	$ 3,871

[1] Accounts written off, net of recoveries.