Schedule II Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts Receivable [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance beginning of period	$ 26,118	$ 12,569	$ 7,889
Additions charged to revenues, costs, expenses	6,847	30,917	8,090
Deductions amounts charged off-net	19,803	17,368	3,410
Balance end of period	$ 13,162	$ 26,118	$ 12,569