Schedule II, Valuation and Qualifying Accounts (Details) (Allowance for Bad Debts, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Allowance for Bad Debts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1	$ 4,911		$ 4,333		$ 4,766
Charged to costs and expenses	4,431		3,988		3,329
Charged to other accounts	1,269	[1]	1,199	[1]	1,310	[1]
Deductions from reserves	4,790	[2]	4,609	[2]	5,072	[2]
Balance at Dec. 31	$ 5,821		$ 4,911		$ 4,333

[1] Recovery of amounts previously written off.

[2] Principally bad debts written off.