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	$ 5,475		$ 4,722		$ 5,380
Charged to costs and expenses	4,137		3,437		2,915
Charged to other accounts	1,089	[1]	1,076	[1]	1,202	[1]
Deductions from reserves	4,862	[2]	3,760	[2]	4,775	[2]
Balance at Dec. 31	$ 5,839		$ 5,475		$ 4,722

[1] Recovery of amounts previously written off.

[2] Principally bad debts written off.