Receivables (Rollforward of Allowance for Doubtful Accounts) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts [Roll Forward]
Balance as of beginning of period	$ 2,838	$ 2,792
Allowance for Loan and Lease Losses, Adjustments, Other	717	1,220
Direct write-offs charged against the allowance	(546)	(1,174)
Balance as of end of period	$ 3,009	$ 2,838