Schedule II, Valuation and Qualifying Accounts (Details) - Allowance for Bad Debts - 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 Jan. 1		$ 4,865	$ 5,128	$ 5,821
Charged to costs and expenses		4,105	3,730	3,947
Charged to other accounts	[1]	952	1,008	1,161
Deductions from reserves	[2]	5,049	5,001	5,801
Balance at Dec. 31		$ 4,873	$ 4,865	$ 5,128

[1] Recovery of amounts previously written off.

[2] Deductions relate primarily to bad debt write-offs.