Schedule II Valuation and Qualifying Accounts (Details) - Reserve for Uncollectible Accounts [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Reserve [Roll Forward]
Balance at beginning of year	$ 305,000	$ 315,000	$ 325,000
Additions - charged to cost and expenses	290,897	291,108	292,248
Additions - recoveries	25,900	38,976	40,681
Deductions	316,797	340,084	342,929
Balance at end of year	$ 305,000	$ 305,000	$ 315,000