SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS (Details) (Allowance for doubtful accounts, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance for doubtful accounts
Movement in valuation and qualifying accounts
Balance at beginning of year	$ 2,979	$ 3,030	$ 3,105
Additions charged to expense	3,840	4,104	4,900
Deductions from allowance	3,970	4,155	4,975
Balance at end of year	$ 2,849	$ 2,979	$ 3,030