SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS (Details) - Allowance for uncollectible Accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of year	$ 238,897	$ 251,734	$ 229,802
Acquisitions	0	0	0
Amounts charged to income	478,365	442,985	422,145
Amounts written off	498,863	455,822	400,213
Balance at end of year	$ 218,399	$ 238,897	$ 251,734