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		$ 6,379	$ 5,888	$ 5,839
Charged to costs and expenses		5,091	6,066	4,655
Charged to other accounts	[1]	1,169	907	1,036
Deductions from reserves	[2]	6,291	6,482	5,642
Balance at Dec. 31		$ 6,348	$ 6,379	$ 5,888

[1] Recovery of amounts previously written off.

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