SCHEDULE II. VALUATION AND QUALIFYING ACCOUNTS - Transactions in the Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance, beginning of year	$ 39,543	$ 43,455	$ 41,051
Provision	13,489	5,136	11,538
Write-offs	(10,623)	(9,048)	(9,134)
Balance, end of year	$ 42,409	$ 39,543	$ 43,455