Schedule II-Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in valuation and qualifying accounts
Balance at beginning of period	$ 1,552	$ 1,853	$ 4,419
Additions:
Bad debt expense	1,213	1,450	1,564
Deductions:
Charge-offs of uncollectible balances	(1,616)	(1,751)	(4,130)
Balance at end of period	$ 1,149	$ 1,552	$ 1,853