Schedule II, Valuation and Qualifying Accounts (Details) - Allowance for Bad Debts - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		$ 5,888	$ 5,839	$ 5,475
Charged to costs and expenses		6,066	4,655	4,137
Charged to other accounts	[1]	907	1,036	1,089
Deductions from reserves	[2]	6,482	5,642	4,862
Balance at Dec. 31		$ 6,379	$ 5,888	$ 5,839

[1] Recovery of amounts previously written off.

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